RB1 and neoplasm: The inactivation of both RB1 alleles in tumour cells was initially detected in association with LOH at the RB1 locus on chromosome 13, and microdeletions or inactivating mutations in the retained allele would be responsible for complete inactivation of the gene (Ishikawa et al, 1991; Hogg et al, 1993; Xu et al, 1993; Gouyer et al, 1994; Henson et al, 1994; Ichimura et al, 1996).