Inclusion Criteria:

          -  Cytologically or histologically and radiologically confirmed solid tumor that is
             inoperable, locally advanced, metastatic, or recurrent.

          -  Dose-Escalation Stage Cohort A (Solid Tumors): The subject has a solid tumor that is
             unresectable or metastatic and for which life-prolonging measures do not exist or
             available therapies are intolerable or no longer effective.

          -  Dose-Escalation Stage Cohort B and Cohort-Expansion Stage Cohorts F and H (Hormone
             Receptor-Positive Breast Cancer): Subjects with breast cancer that is hormone
             receptor-positive (estrogen receptor positive [ER+] and/or progesterone receptor
             positive [PR+]) and negative for human epidermal growth factor receptor 2 (HER-2
             negative [HER-2-]) and who have documented radiographic disease progression during or
             following their last systemic anticancer therapy for inoperable locally advanced or
             metastatic disease.

          -  Dose-Escalation Stage Cohort C and Cohort-Expansion Stage Cohorts G and I (Metastatic
             Castration-Resistant Prostate Cancer): Subjects with adenocarcinoma of the prostate.
             Note: Neuroendocrine differentiation and other histological features are permitted if
             adenocarcinoma is the primary histology.

          -  Cohort-Expansion Stage Cohort D (Triple Negative Breast Cancer): Subjects with breast
             cancer that is negative for HER-2, estrogen receptors, and progesterone receptors, and
             who have documented radiographic disease progression during or following their last
             systemic anticancer therapy for inoperable locally advanced or metastatic disease.

          -  Cohort-Expansion Stage Cohort E (Epithelial Ovarian Cancer): Subjects with epithelial
             ovarian cancer, including primary peritoneal cancer (PPC) and fallopian tube cancer
             (FTC) who have platinum-resistant disease following treatment with a
             platinum-containing chemotherapy. Ovarian borderline epithelial tumors (low malignant
             potential) are excluded.

          -  Expansion Cohorts: Subjects must have measurable disease per RECIST 1.1 as determined
             by the Investigator.

          -  Tumor tissue material (archival or fresh tumor tissue [if it can be safely obtained]).

          -  Recovery to baseline or ≤ Grade 1 severity (Common Terminology Criteria for Adverse
             Events version 5 [CTCAE v5]) from AEs.

          -  Eastern Cooperative Oncology Group (ECOG) Performance Status of 0-1.

          -  Adequate organ and marrow function.

          -  Sexually active fertile subjects and their partners must agree to use medically
             accepted methods of contraception.

          -  Female subjects of childbearing potential must not be pregnant at screening.

        Exclusion Criteria:

          -  Receipt of XL102 or any other selective CDK7 inhibitor.

          -  Receipt of any cytotoxic chemotherapy therapy or anticancer antibody therapy within 21
             days before first dose of study treatment.

          -  Receipt of any type of small molecule kinase inhibitor within 2 weeks before first
             dose of study treatment.

          -  Receipt of any anticancer hormonal therapy within 2 weeks or within 5 half-lives of
             the agent, whichever is shorter, before first dose of study treatment.

          -  HR+BC subjects enrolled in the Combination Cohorts B and H receiving fulvestrant prior
             to first dose of study treatment are allowed to continue with their fulvestrant
             treatment.

          -  Metastatic CRPC subjects enrolled in the Combination Cohorts C and I receiving
             abiraterone prior to first dose of study treatment are allowed to continue with their
             abiraterone treatment.

          -  Radiation therapy within 14 days before first dose of study treatment. Subjects with
             clinically relevant ongoing complications from prior radiation therapy are not
             eligible.

          -  Known brain metastases or cranial epidural disease unless adequately treated with
             radiotherapy and/or surgery (including radiosurgery) and stable for at least 4 weeks
             before first dose of study treatment.

          -  Concomitant use of certain medications.

          -  Uncontrolled, significant intercurrent or recent illness.

          -  Corrected QT interval calculated by the Fridericia formula (QTcF) > 480 ms per
             electrocardiogram (ECG).

          -  Pregnant or lactating females.

          -  Diagnosis of another malignancy within 2 years before first dose of study treatment,
             except for superficial skin cancers, or localized, low grade tumors deemed cured and
             not treated with systemic therapy.