In the PALOMA-1 phase II clinical trial (NCT00721409) with hormone receptor-positive (HR+)/HER2- breast cancer patients, progression-free survival (PFS) after treatment with either 2.5 mg letrozole daily or 2.5 mg letrozole daily and 125 mg palbociclib in 3 week on/1 week off cycles was monitored. This evidence concerns the gene ERBB2 and breast cancer.