The formation of the sex vesicle, or XY body, during male meiosis and pairing of 
the sex chromosomes are thought to be essential for successful spermatogenesis. 
Despite its cytological discovery a century ago, the mechanism of XY body 
formation, particularly heterochromatinization of the sex chromosomes, has 
remained unclear. The HP1 class of chromobox genes are thought to encode 
proteins involved in the packaging of chromosomal DNA into repressive 
heterochromatin domains, as seen, for example, in position-effect variegation. 
Study of the distribution of a murine HP1-like chromodomain protein, M31, during 
spermatogenesis revealed spreading from the tip of the XY body in mid-stage 
pachytene spermatocytes to include the whole of the XY body in late-pachytene 
spermatocytes. We also demonstrate that the formation of the XY body during 
spermatogenic progression in neonatal mice coincides with the expression of a 
novel nuclear isoform of M31, M31(p21). These results support the view that a 
common mechanistic basis exists for heterochromatin-induced repression, homeotic 
gene silencing, and sex-chromosome inactivation during mammalian 
spermatogenesis.