RB1 and cancer: Surprisingly, RB1 protein loss was less frequent than RNASEH2B protein loss; 5 of 93 (5.4%) mCRPC biopsies had complete RB1 loss with many biopsies (over 60%) having a smaller proportion of cancer cells with RB1 loss (under 20% cells with RB1 loss), although heterogeneous loss of RB1 in mCRPC was also confirmed (Figure 4A).