How mammals pack their sperm: a variant matter.

Producing competent gametes is essential for transmitting genetic information throughout generations. Spermatogenesis is a unique example of rearrangements of genome packaging to ensure fertilization. After meiosis, spermatids undergo drastic morphological changes, perhaps the most dramatic ones occurring in their nuclei, including the transition into a protamine-packaged genome. In this issue of Genes & Development, Montellier and colleagues (pp. 1680-1692) shed new light on the molecular mechanisms regulating this transition by ascribing for the first time a function to a histone variant, TH2B, in the regulation of this process.