NR2E3, a photoreceptor, specific nuclear receptor (PNR), represses cone-specific genes and activates several rod-specific genes. In humans, mutations in NR2E3 have been associated with the recessively-inherited enhanced short-wavelength sensitive Scone syndrome (ESCS) and, recently, with autosomal dominant (ad) retinitis pigmentosa (RP) (adRP). In the present work, we describe two additional families affected by adRP that carry a heterozygous c.166G > A (p.G56R) mutation in the NR2E3 gene. Functional analysis determined the dominant negative activity of the p.G56R mutant protein as the molecular mechanism of adRP Interestingly, in one pedigree, the most common causal variant for ESCS (p.R311Q) cosegregated with the adRP linked p.G56R mutation, and the compound heterozygotes exhibited an ESCS-like phenotype, which in 1 of the 2 cases was strikingly "milder" than the patients carrying the p.G56R mutation alone. Impaired repression of cone-specific genes by the corepressors atrophin-1 (dentatorubral-pallidoluysian atrophy [DRPLA] gene product) and atrophin-2 (arginine-glutamic acid dipeptide repeat [RERE] protein) appeared to be a molecular mechanism mediating the beneficial effect of the p.R311Q mutation. Finally, the functional dominance of the p.R311Q variant to the p.G56R mutation is discussed. Hum Mutat 30, 342-351, 2009. (C) 2008 Wiley-Liss, Inc.