Differential gene expression is what allows cells with the same genetic material to perform different functions, and is therefore a fundamental question in the field of developmental biology. Although all of the steps involved in gene expression can, in principle, be regulated (like RNA processing and protein modifications), for most genes, the initial act of transcription is the primary point of control. During my thesis, I have focused my research on understanding how genes can be selectively regulated. For this work, I have used the Drosophila homeotic gene, Abdominal-B as my model system. Abd-B is the homeotic gene that controls the segmental identity of the most-posterior abdominal segments. Its transcription is controlled in a parasegmental manner by a large cis-regulatory domain, spanning over 80kb.