The conserved bromodomain motif binds to acetylated lysines in histones,but although some bromodomain-containing proteins are implicated in chromatin remodelling, the in vivo roles of most are poorly understood. Now, on p. 3507, Shang and colleagues report that Brdt, a testis-specific member of the BET subfamily of double-bromodomain-containing proteins, is essential for male germ cell differentiation. The researchers report that mice homozygous for a mutant allele of Brdt that lacks the first bromodomain(BrdtΔBD1) are viable but the males are infertile. The morphologically abnormal sperm that these animals make lack the foci of heterochromatin at the perinuclear envelope seen in elongating wild-type spermatids. Furthermore, the researchers report, there is increased expression of testis-specific histone H1t in BrdtΔBD1/ΔBD1testes, and Brdt protein (but not BrdtΔBD1 protein)associates with the H1t promoter. These results suggest that Brdt is involved in the chromatin condensation that occurs during the late stages of spermatogenesis - interestingly, some infertile but otherwise healthy men have mutations in the human BRDT gene.