Following identification of human RP patients who were doubly heterozygous for a mutation in PRPH2 (RDS) and a null mutation in ROM1 (Dryja et al., 1997; Kajiwara et al., 1994), a similar digenic mutant mouse model showed faster photoreceptor degeneration compared with rds mutation alone, and a positive correlation was observed between the rate of photoreceptor loss and the extent of OS disorganization (Kedzierski et al., 2001). The gene discussed is PRPH2; the disease is retinitis pigmentosa 1.