Background A combination of histone modifications, a ‘histone epigenetic code’, is created and altered by specific enzymes, and recognized by proteins that bind to these modifications through specific domains [1]. These proteins modify the chromatin environment for precise transcriptional regulation. Some of these well studied chromatin remodeling complexes include the Polycomb and the Trithorax Group Proteins (PcG and Trx-G). The two groups are primarily antagonistic and control patterning of the body during embryogenesis, through the regulation of gene expression. The Polycomb Repressive Complexes mediate the initiation of gene repression and its maintenance. The Trx-G proteins are involved in maintaining the activation of gene expression [1]. In Drosophila, one of the earliest developmental decisions made in the embryo is that of determining its sex. This process involves regulating the expression of the X chromosome sensing promoter of Sex-lethal (Sxl) at its establishment promoter, Sxlpe [2]. Using this sensitive system which differentiates one versus two X chromosomes, our lab has shown that heterochromatin proteins are required for proper Sxlpe regulation. We found that Heterochromatin Protein 1 a (HP1a) plays both a repressive and activating role in regulating Sxlpe [3].