We present information obtained from the occurrence of psoriasis and its related disease pityriasis rubra pilaris (PRP) in a large, five-generation kindred. Using a genome-wide scan and single nucleotide polymorphism (SNP) fine mapping, the psoriasis/PRP locus was mapped to chromosome 17q terminus, a region close to but distinct from the 17q PSOR2 locus. Moreover, we sequenced the candidate genes in this region, and identified CD7 as the susceptibility gene in this family. Compositions and methods of use are provided for the prediction, diagnosis, disease monitoring and therapeutic development of psoriasis and pityriasis rubra pilaris (PRP).