This drug was initially investigated as monotherapy in a phase II, single-arm study for metastatic breast cancer patients expressing Rb protein in their tissues, with the primary objective to evaluate tumor response and with some of the secondary objectives including progression-free survival (PFS) and expression of Rb, p16Ink4a loss, and cyclin D1 gene (CCND1) amplification [46]. Here, RB1 is linked to neoplasm.