Like most human solid tumors, breast cancers harbor frequent alterations in the pRb pathway, including CCND1 overexpression in 45% (Buckley et al. 1993), p16INK4A loss in 49% (Geradts and Wilson 1996), and pRb loss in 6% of breast tumors (Geradts and Wilson 1996). Here, RB1 is linked to breast cancer.