| Histone variants such as histone H3 . 3 , H2AX , and macroH2A appear to play key roles in the various stages of spermiogenesis , in addition to the specifically modulated acetylation of histone H 4 ( acH 4 ) , ubiquitination of histones H2A and H2B ( uH2A , uH2B ) , and phosphorylation of histone H 3 ( H3p ) . ^^^ |