RB1 and colorectal cancer: The frequency of pRb expression is in agreement with earlier observations that approximately one-third of colorectal cancers have increased copy numbers of one Rb1 allele [28,29], a 2- to 5- fold increase in Rb protein and mRNA levels compared with adjacent normal mucosa [30], as well as a 2-fold increase of the abundance of pRb and its phosphorylation status compared with normal mucosa [31].