A cDNA named calmin of ∼3.2 kb was isolated by RNA differential display applied to developing mouse skin. Calmin cDNA encodes 1021 amino acids with two calponin homology (CH) domains in tandem on the N-terminal side and a transmembrane domain on the C-terminal side. The region covering the CH domains showed a high level of homology with β-spectrin, α-actinin, and dystrophin. Among the proteins with the tandem CH domains, calmin is unique in having a transmembrane domain. Three alternative splicing sites were identified at the 3′-side of calmin, giving rise to polymorphic protein products with or without the transmembrane domain. The calmin transcript was detected in adult testis, liver, kidney, and large intestine; the expression in testis was far stronger than that in the other tissues. In situ hybridization and immunostaining revealed that calmin was expressed in maturing spermatogenic cells at later stages. Human calmin cDNA was also isolated, and its exon/intron organization was determined.