HOAP was originally identified as a component of an ORC-containing multi-protein complex of Heterochromatin Protein 1 (HP1) from early Drosophila embryos. HOAP immunostaining showed prominent association of it with telomeres, and mutants for HOAP (cav1) showed it functions along with HP1 in forming a telomere capping complex that prevents telomeric fusions. Weaker HOAP immunostaining is also observed in regions of pericentric heterochromatin and euchromatin. To examine the role of HOAP at these non-telomeric sites, we applied Affymetric Drosophila Genome Arrays to undertake a microarray expression profiling study of genes that are mis-expressed in cav1 mutant larvae. The data from four publicly available databases were used to assess the normal expression patterns of the affected genes. We found that the majority (67%) of genes with decreased expression levels in cav1 mutants (log2R< -2.0, pvalue≤ 0.01) have normally testis-specific expression. These results could indicate a role of HOAP in testis-specific gene expression. Alternatively they could reflect reduced male viability due to the loss of HOAP, which resulted in the under-representation of males in the cav1 larval sample. The latter hypothesis is supported by the observation of 2.8-fold under-representation of males in cav1 larvae when I used a yellow+-marked X chromosome to differentially mark male and female cav1 larvae. Thus, this project is focused on determining and characterizing the cause of the reduced male viability. Here I report a role for both HOAP and HP1 in regulating the establishment promoter, SxlPe, of the sex determination masterswitch, Sex lethal (Sxl). Female-specific activation of SxlPe is essential to females as it provides SXL protein to initiate productive female-specific splicing of the late Sxl transcripts which are transcribed in both sexes. We find inappropriate firing of SxlPe and splicing of Sxl transcripts in male cav mutants, whereas mutants for HP1 display Sxl splicing defects in both sexes. Both proteins are associated with SxlPe sequences. In embryos from HP1 mothers and Sxl mutant fathers, female viability and RNA polymerase II recruitment to SxlPe is severely compromised. Our genetic and biochemical assays suggest a repressing activity for HOAP and both activating and repressing roles for HP1 at SxlPe.