Experimental data show that in vitro methylation of the RB1 promoter region reduces pRB expression (Ohtani-Fujita et al, 1993) and unilateral retinoblastoma frequently shows loss of pRB expression associated with aberrant methylation of CpG island within the RB1 promoter region (Stirzaker et al, 1997). Here, RB1 is linked to retinoblastoma.