Inclusion Criteria:

          1. Female

          2. Age ≥ 18 years

          3. Histological diagnosis of invasive lobular breast adenocarcinoma that is ER+, and
             HER2- as per the updated American Society of Clinical Oncology (ASCO) - College of
             American Pathologists (CAP) guidelines according to local testing.

          4. Multifocal unilateral or bilateral breast adenocarcinoma tumours are allowed if all
             tested foci are lobular, ER+ and HER2-.

               -  ER positive (ER+ is defined as having an IHC of 1% or more and/or an Allred of 3
                  or more and HER2-).

               -  HER2 negative (HER2- is defined as having an IHC of 0 or 1+ without ISH OR IHC 2+
                  and ISH non-amplified with ratio less than 2.0 and if reported, average HER2 copy
                  number < 4 signals/cells OR ISH non-amplified with ratio less than 2.0 and if
                  reported, average HER2 copy number < 4 signals/cells [without IHC]);

          5. A primary non metastatic or locally advanced tumour of 15 mm or more, cN0 or cN1
             without prior treatment candidate for preoperative treatment.

          6. ECOG Performance Status (PS) 0 or 1.

          7. Adequate Bone Marrow Function including:

               -  Absolute Neutrophil Count (ANC) ≥1500/μL or ≥1.5x109/L;

               -  Platelets ≥100000/μL or ≥100 x 109/L;

               -  Haemoglobin ≥ 9 g/dL.

          8. Adequate Renal Function including:

             o Serum creatinine ≤ 1.5 x upper limit of normal (ULN) or estimated creatinine
             clearance ≥ 60 ml/min as calculated using the method standard for the institution.

          9. Adequate Liver Function, including all of the following parameters:

               -  Total serum bilirubin ≤ 2.0 x ULN unless the subject has documented Gilbert
                  syndrome

               -  Aspartate and Alanine Aminotransferase (AST and ALT) ≤ 3 x ULN;

         10. Signed Informed Consent form (ICF) obtained prior to any study related procedure.

         11. Completion of all necessary screening procedures within 28 days prior to enrolment.
             Biopsies at screening must have been obtained up to max 6 weeks before the beginning
             of treatment.

         12. Subject is willing and able to comply with the protocol for the duration of the study
             including treatment and scheduled visits and examinations.

         13. Women who are not postmenopausal or have not undergone hysterectomy must have
             documented negative pregnancy test (serum) within 28 days prior to enrolment.

         14. Women of childbearing potential and their partners, who are sexually active, must
             agree to use one highly effective form of contraception (see protocol section 6.6.1)
             from the signing of the ICF until at least 5 weeks after last administration of
             entrectinib, or they must totally/truly abstain from any form of sexual intercourse.
             Use of oral hormonal contraceptive agents in this study is not permitted.

             Inclusion criterion applicable to FRANCE only:

         15. Subject is affiliated to the French Social Security System.

        Exclusion Criteria:

          1. Clinical T4 disease including inflammatory breast cancer and/or cN3.

          2. Prior history of invasive cancer in the past 5 years except basal or squamous cell
             carcinoma of skin that has been definitively treated.

          3. Known hypersensitivity to the study drugs or excipients.

          4. Any illness or medical condition that is unstable or could jeopardize the safety of
             the subject or her compliance with study requirements.

          5. Subjects unable to swallow oral medications.

          6. Prior intake of letrozole, any ROS1 inhibitor, any TRK inhibitor or anticancer therapy
             (including endocrine therapy).

          7. Concurrent treatment with strong or moderate CYP3A inhibitor.

          8. Concurrent treatment with any of the drugs not permitted, i.e. strong CYP3A inducers
             and drugs known to cause QTc interval prolongation.

          9. LVEF ≤ 55% measured by echo or MUGA

         10. QTc exceeding 450 msec, history of prolonged QTc interval prolongation; risk factors
             for torsade de pointes; other concomitant medications that may prolong QTc; family or
             personal history of long or short QT syndrome, Brugada syndrome or known history of
             QTc prolongation, or Torsade de Pointes (TdP).

         11. Pregnant or lactating women.

         12. Known interstitial lung disease, interstitial fibrosis, or history of tyrosine kinase
             inhibitor-induced pneumonitis

         13. Peripheral neuropathy ≥ Grade 2

         14. Active gastrointestinal disease (e.g., Crohn's disease, ulcerative colitis, or short
             gut syndrome) or other malabsorption syndromes that would reasonably impact drug
             absorption.

        Exclusion criterion applicable to France only 14. Vulnerable persons according to the
        article L.1121-6 of the CSP, adults who are the subject of a measure of legal protection or
        unable to express their consent according to article L.1121-8 of the CSP.