This is a first-in-human study of MEN2312, a lysine acetyltransferase 6 (KAT6) inhibitor, in adult participants with advanced breast cancer.

Key Inclusion Criteria:

• Participant has advanced (locoregionally recurrent or metastatic) breast cancer not amenable to curative therapy.
• Presence of genetic alterations in PIK3CA/AKT1/PTEN in participants' tumor tissue.
• Participant must have received at least 1 prior line of endocrine therapy for advanced/metastatic disease or participant who has radiological evidence of breast cancer recurrence or progression during or within 12 months from the end of adjuvant treatment with endocrine therapy, as these participants are considered as first-line relapsed participants.
• Progression on previous cyclin-dependent kinase 4 and 6 inhibitor treatment in combination with fulvestrant or aromatase inhibitor is required.

Key Exclusion Criteria:

• Active or newly diagnosed central nervous system metastases.
• Participants with advanced, symptomatic visceral spread, who are at risk of life-threatening complications in the short term, including massive uncontrolled effusions (peritoneal, pleural, pericardial), pulmonary lymphangitis, or liver involvement >50%.
• Participants with any toxicities related to prior radiation therapy that have not resolved to baseline or to National Cancer Institute Common Terminology Criteria for Adverse Events v5.0 Grade ≤1, except alopecia and peripheral sensory neuropathy (Grade ≤2).

Note: Other inclusion/exclusion criteria may apply.