CLPH (Casein-Like phosphoprotein) is a recently discovered, evolutionarily conserved, testis-specific cytoplasmic protein expressed exclusively in postmeiotic germ cells in rat spermatogenesis. CLPH is a novel casein kinase 2-phosphorylated disordered and calcium-binding protein that displays strong variation in relative expression levels during spermatogenesis. It is frequently associated with the mitochondrial inner membrane during the last steps of spermatid differentiation and is involved in the extremely complex structural rearrangements occurring in haploid germ cells during spermiogenesis.