During development, differential gene expression governs generation of the cellular diversity required for establishment of pattern formation and organogenesis. An important mechanism to ensure proper temporal and spatial gene expression throughout development is regulation at the transcriptional level. Regulation of transcription is a highly dynamic cellular process involving a complex array of protein-protein and protein-DNA interactions. Studies in yeast (Saccharomyces cerevisiae) and fruit fly (Drosophila melanogaster) implicate a number of regulatory proteins, both general and gene-specific, as well as regulatory mechanisms integral to this process. Key classes of factors in transcriptional regulation are those that modulate chromatin structure. Chromatin remodeling is of fundamental importance because it affects the conformation and position of nucleosomes, which, if situated at promoter elements, can prevent RNA polymerase II holoenzyme from accessing DNA and initiating transcription. Mammalian SWI/SNF-related complexes utilize either brahma (Brm) or brahma-related gene 1 (Brgl) catalytic subunits to remodel nucleosomes in an ATP-dependent manner. A Brgl null mutation was produced and homozygotes die at implantation, but, because BrgI mRNA and protein are expressed at high levels in the oocyte, experiments have been designed to determine whether Brgl is required for zygotic genome activation. Interpretation of the peri-implantation phenotype has also been complicated by the fact that Brgl is the catalytic subunit of two distinct, but related, chromatin-remodeling complexes. Therefore, mice deficient for complex-specific subunits will be created to assess the relative importance of each complex. Finally, to move beyond implantation and elucidate Brgl function at later stages of embryonic and post-natal development, ENEJ mutagenesis and Cre-loxP conditional gene targeting is being used to examine its role in the development of the hematopoietic system.