RB1 and neoplasm: The discrepancies observed among the rates of LOH at 13q (30% of cases), RB1 inactivating mutations (5–12% of cases), homozygous RB1 deletions (3% of tumours), and loss of RB1 expression detected by immunohistochemical analysis (up to 27% of samples), strongly suggest that other molecular mechanisms may participate in the inactivation of this gene; moreover, loss of RB1 expression does not always correlate with LOH at the RB1 locus (Henson et al, 1994; Ichimura et al, 1996; Nakamura et al, 1996; Ueki et al, 1996; Burns et al, 1998).