Protamines substitute for histones in the chromatin of sperm during the haploid phase of spermatogenesis. They compact sperm DNA into a highly condensed, stable and inactive complex. Octopus spermiogenesis is characterized by a double nuclear protein transition: Histones are first replaced by P1, which allows the chromatin to adopt a shape that is not as relaxed as with histones. The majority of P1 is later replaced by P2, forming a compact chromatin. P2 is the main protamine of sperm. Testis. P2 is phosphorylated in immature sperm. It is dephosphorylated in mature sperm allowing a stronger interaction with DNA.