Heterochromatin is DNA that is tightly packed with histones and other proteins and its stability is important for maintaining genome integrity and function. Heterochromatin is generally gene-poor and provides a repressive environment for transcription. Drosophila myb is the fly homologue of the c-Myb proto-oncogene and contributes to the condensation of euchromatin, the promotion of cell division, and the replication of polytene chromosomes under certain situations. Previous results from our lab have shown that when Myb is absent, there is an increase in expression of a heterochromatin-incorporated lacZ gene in polytene salivary gland chromosomes. These results suggest three possible mechanisms for how Myb might be affecting lacZ expression: first, Myb could be directly stabilizing the heterochromatin such that removal of myb leads to derepression of the lacZ; second, Myb could be directly repressing genes embedded in heterochromatin, and its removal would also lead to lacZ derepression; thirdly, Myb could be inhibiting the replication of heterochromatic DNA, thus preventing large amounts of lacZ from being transcribed due to fewer strands of DNA being synthesized. In this study, we tested the third hypothesis, namely to determine whether Drosophila Myb is important for repressing the replication of various types of heterochromatin. We isolated DNA from salivary glands of both myb mutant and control male larvae and determined through array-based comparative genomic hybridization that heterochromatic regions are over-replicated in the myb mutant. These results indicated that myb is controlling the expression of heterochromatin-embedded genes through regulation of the number of gene copies. "