RB1 and neoplasm: Paired tumor biopsies at screening and cycle 2 day 1 were collected from eight patients with HR+/HER2− mBC treated with 25 mg twice daily PF-06873600 monotherapy (part 1A) or in combination with fulvestrant (parts 1B and 2A) and analyzed for PD biomarker (pRb and Ki67 expression) changes by IHC.