To identify the gene causing diffuse palmoplantar keratoderma in a Chinese pedigree.Four normal individuals and 3 patients in a diffuse palmoplantar keratoderma family and 10 unrelated control samples were recruited. The hotspot of the mutations of keratin 9 gene was analyzed by polymerase chain reaction and direct sequencing.We found a G485A transition in ke ratin 9 gene, resulting in the substitution of glutamine for arginine at codon 162 in this diffuse palmoplantar keratoderma family. The mutation was not found in the 10 unrelated control samples and 4 normal individuals.The mutation G485A found in keratin 9 gene is the disease-causing mutation in the diffuse palmoplantar keratoderma family.