This study is a randomized, double-blind, placebo-controlled international phase 2 trial in patients with HRG+, HR+, HER2- metastatic breast cancer that has progressed following treatment with no more than 2 prior therapies, one of which must have been a CDK inhibitor. All patients will be screened for heregulin using central testing, and eligible patients will be randomized to receive either seribantumab + fulvestrant or placebo + fulvestrant. Disease status will be assessed according to RECIST v 1.1 to support the primary endpoint.