-  INCLUSION CRITERIA:

        All phases and cohorts

          -  Subjects must not have received chemotherapy or undergone major surgery within 2 weeks
             and radiotherapy within 24 hours prior to cycle 1 day 1.

          -  Age greater than or equal to 18 years. Because no dosing or adverse event data are
             currently available on the use of sacituzumab govitecan in combination with and
             berzosertib in participants <18 years of age, children are excluded from this study,
             but will be eligible for future pediatric trials.

          -  ECOG performance status less than or equal to 2

          -  Participants must have adequate organ and marrow function as defined below:

               -  leukocytes greater than or equal to 3,000/mcL

               -  Hemoglobin greater than or equal to 9.0g/dL

               -  absolute neutrophil count greater than or equal to 1,500/mcL

               -  platelets greater than or equal to 100,000/mcL

               -  total bilirubin within normal institutional limits

               -  AST(SGOT)/ALT(SGPT) less than or equal to 2.5 X institutional upper limit of
                  normal

               -  creatinine within normal institutional limits

        OR

        --creatinine clearance greater than or equal to 30 mL/min/1.73 m2 for participants with
        creatinine levels above institutional normal

        (calculated by Cockcroft-Gault formula).

          -  Participants with neurologically stable brain metastases defined as asymptomatic
             metastasis, or treated metastasis having no evidence of progression or hemorrhage for
             at least 2 weeks after treatment (including brain radiotherapy) may be included.
             Participants must be off any systemic corticosteroids for the treatment of brain
             metastases for at least 7 days prior to enrollment.

          -  Participants with previously treated with topoisomerase 1/2 inhibitors can be
             enrolled.

          -  The effects of the combined study drugs on the developing human fetus are unknown. For
             this reason and because study agents as well as other therapeutic agents used in this
             trial are known to be teratogenic, women of child-bearing potential and men must agree
             to use adequate contraception (hormonal or barrier method of birth control;
             abstinence) prior to study entry, while taking the study drugs and for 6 months after
             the administered study drug (berzosertib or sacituzumab govitecan). Should a woman
             become pregnant or suspect she is pregnant while she or her partner is participating
             in this study, she should inform her treating physician immediately.

          -  Ability of subject to understand and the willingness to sign a written informed
             consent document.

        Phase I

          -  Participants with histologically or cytologically confirmed advanced solid tumors with
             progression on at least one prior chemotherapy.

          -  Known HRD cancer and documented evidence of at least ONE or MORE of the following:

               -  Pathogenic or likely pathogenic somatic mutation or inactivating alteration of a
                  gene involved in homologous recombination (BRCA1, BRCA2, ATM, BRIP1, BARD1,
                  CDK12, CHEK1, CHEK2, FANCL, PALB2, PPP2R2A, RAD51B, RAD51C, RAD51D, or RAD54L)
                  repair in the tumor. Local testing in Clinical Laboratory Improvement Act
                  (CLIA)-certified laboratory will be accepted. No

        variants of uncertain significance (VUS) will be allowed.

          -  If this alteration is identified in a circulating tumor deoxyribonucleic acid (ctDNA)
             assay, the variant-allele fraction must be > 20% to indicate relevance to predominant
             tumor clone

          -  Mutation in one or more other genes involved in homologous DNA recombination repair in
             the tumor may be included at investigator's

        discretion

        ---Homologous recombination repair deficiency by genomic signature in the tumor by
        BROCA-HR, Foundation One or equivalent assay

        --Presence of pathogenic or likely pathogenic germline mutation/variant in BRCA1, BRCA2,
        ATM, BRIP1, BARD1, CDK12, CHEK1, CHEK2, FANCL, PALB2, PPP2R2A, RAD51B, RAD51C, RAD51D, or
        RAD54L. Germline mutations in other HR genes will be considered at investigator's
        discretion. Local testing in Clinical Laboratory Improvement Act (CLIA)-certified
        laboratory will be accepted. No variants of uncertain significance (VUS) will be allowed.

          -  Participants must have measurable disease, per RECIST 1.1.

          -  Participants must have at least one lesion deemed safe to biopsy and be willing to
             undergo mandatory biopsies (Pre-Tumor, On-Treatment, PD). Up to 10 participants with
             non-biopsiable disease may be enrolled.

          -  Participants should have demonstrated progressive disease while taking a PARPi as a
             previous therapy or within 6 months of completing PARPi therapy. Response to prior
             PARPi is not required.

          -  Participants may have received chemotherapy in the interval between PARPi and
             enrollment.

        Phase II SCLC

          -  Recurrent histologically or cytologically confirmed SCLC after at least one prior
             platinum-based therapy.

          -  Participants must have measurable disease, per RECIST 1.1.

          -  Participants must have at least one lesion deemed safe to biopsy and be willing to
             undergo mandatory biopsies (Pre-Tumor, On-Treatment, PD). Up to 10 participants with
             non-biopsiable disease may be enrolled.

        EXCLUSION CRITERIA:

          -  Participants who are receiving any other investigational agents or concurrent systemic
             anti-cancer therapies.

          -  History of allergic reactions attributed to compounds of similar chemical or biologic
             composition to study drugs or other agents used in study.

          -  Participants with symptomatic brain metastasis.

          -  Participants who require treatment with strong inhibitors or inducers of CYP3A or with
             UGT1A1 inhibitors during the planned period of investigational treatment with
             sacituzumab govitecan.

          -  Participants known to be homozygous for the UGT1A1*28 variant allele with severely
             reduced UGT1A1 activity.

          -  Uncontrolled intercurrent illness including, but not limited to, ongoing or active
             infection, symptomatic congestive heart failure, unstable angina pectoris, cardiac
             arrhythmia, or psychiatric illness/social situations that would limit compliance with
             study requirements.

          -  Pregnant women are excluded from this study because study drugs have the potential for
             teratogenic or abortifacient effects. Because there is an unknown but potential risk
             for adverse events in nursing infants secondary to treatment of the mother with study
             drugs, breastfeeding should be discontinued if the mother is treated with the study
             drugs. These potential risks may also apply to other agents used in this study.

          -  Participants with myelosuppressive disorders or acute myeloid leukemia.

          -  HIV positive participants with the following exception: Patients with long-standing
             (>5 years) HIV on antiretroviral therapy > 1 month (undetectable HIV viral load and
             CD4 count > 150 cells/microL) may be eligible if the principal investigator determines
             no anticipated clinically significant drug-drug interactions.

          -  Participants with evidence of chronic hepatitis B virus (HBV) infection, unless the
             HBV viral load must be undetectable on suppressive therapy, if indicated.

          -  HCV infected participants with the following exceptions: Participants with a history
             of hepatitis C virus (HCV) infection must have been treated and cured. For
             participants with HCV infection who are currently on treatment, they are eligible if
             they have an undetectable HCV viral load.