The male-specific lethal (MSL) complex upregulates the single male X chromosome 
to achieve dosage compensation in Drosophila melanogaster. We have proposed that 
MSL recognition of specific entry sites on the X is followed by local targeting 
of active genes marked by histone H3 trimethylation (H3K36me3). Here we analyze 
the role of the MSL3 chromodomain in the second targeting step. Using ChIP-chip 
analysis, we find that MSL3 chromodomain mutants retain binding to chromatin 
entry sites but show a clear disruption in the full pattern of MSL targeting in 
vivo, consistent with a loss of spreading. Furthermore, when compared to wild 
type, chromodomain mutants lack preferential affinity for nucleosomes containing 
H3K36me3 in vitro. Our results support a model in which activating complexes, 
similarly to their silencing counterparts, use the nucleosomal binding 
specificity of their respective chromodomains to spread from initiation sites to 
flanking chromatin.