RB1 and neoplasm: Nonetheless, the finding of tumours displaying LOH at the RB1 locus without any abnormality in the remaining RB1 allele, but with the absence of pRB expression (Borg et al, 1992; Cooney et al, 1996; Ueki et al, 1996; Burns et al, 1998; Simpson et al, 1999), supports the existence of mutations in regions such as promoter or introns, which are frequently not explored or escape SSCP studies.