<p>Nuclear transition protein 1 (TP1) is one of the spermatid-specific proteins [<cite idref="PUB00001393"/>]. TP1 is a basic protein well conserved in mammalian species. In mammals, the second stage of spermatogenesis is characterised by the conversion of nucleosomal chromatin to the compact, non-nucleosomal and transcriptionally inactive form found in the sperm nucleus. This condensation is associated with a double-protein transition. The first transition corresponds to the replacement of histones by several spermatid-specific proteins (also called transition proteins) which are themselves replaced by protamines during the second transition.</p><p>This entry represents a heptapeptide located in positions 28 to 34, which contains five basic residues (Arg or Lys) as well as a tyrosine that could be essential for the destabilisation of chromatin by intercalating between the bases of DNA.</p>