Psds1 is a modifier of Irf2 and interacts with Psds3 on chromosome 16. Animals homozygous for the Irf2tm1Mak mutation, homozygous for C57BL/6 alleles at Psds1, and heterozygous for C57BL/6 and BALB/c alleles at Psds3 displayed the severest skin disease.