AT-motif binding factor 1 (ATBF1) binds to the AT-rich core sequence element in the human a-fetoprotein enhancer (1). Alternative splicing generates the ATBF1-A and ATBF1-B (2,3). While ATBF1-A contains a 920-amino acid extension at the N-terminus, both ATBF1-A and ATBF1-B contain 4 DNA-binding homeobox domains (2,3). Additionally, ATBF1-A contains 23 zinc finger motifs while ATBF1-B contains 18 zinc finger motifs (1–3). The N-terminal extension unique to ATBF1-A has transcriptional repressor activity (4). In the small intestine, ATBF1-A inhibits expression of the brushborder enzyme aminopeptidase-N through direct binding to the AT motif element (5). Besides functioning in transcription regulation, ATBF1 also functions in ATPase activity (6). ATPase activity associated with ATBF1-A is DNA/RNA-dependent and requires both homeobox domains and zinc finger motifs (6). ATBF1 is highly expressed in spleen and brain tissues (7). The gene encoding human ATBF1 maps to chromosome 16q22.3-q23.1 (8).