RB1 and pachyonychia congenita: The RB1 gene has been implicated in the pathogenesis of PC, as allelic loss of RB1 has been observed in ∼30%–100% of PCs and decreased RB1 expression has been reported in >85% of PCs (Figure 3) (Cryns et al., 1994b; Dotzenrath et al., 1996; Szijan et al., 2000).