Expression of housekeeping genes is essential and necessary for cell proliferation. The products of housekeeping genes have been strongly conserved during evolution, an indication that they are essential for living organisms. Human MAZ (Myc-associated zinc finger protein) and its mouse homolog, Pur-1 (purine binding factor-1), are transcription factors that bind to the GA box (GGGAGGG). We have characterized the gene structures, patterns of expression, and promoters of both the human gene for MAZ andthe mouse gene for Pur-1. We have also shown that the genes for MAZ and Pur-1 are typical housekeeping genes. One unusual feature of the promoter of the MAZ gene is that it is almost composed of G and C (G+C content, 88.4percent). MAZ and Pur-1 regulate a number of target genes, including the c-myc gene, the gene for complement 2, genes for insulin I and II, the gene for serotonin receptor, the gene for CD4, the gene for nitric-oxide synthase, and the gene for adenovirus major late protein. MAZ and Pu r-1 have roles in both the initiation and termination of transcription, and they appear to be involved in the etiology of some diseases, such as insulin-dependent diabetes mellitus and chronic myelogenous leukemia. SAF, the rabbit homolog of MAZ, was isolated recently. It is an activator of the gene for serum amyloid A, which is involved in several chronic inflammatory diseases. The multiple functional roles of MAZ and Pur-1 indicate that their housekeeping functions are concerned mainly with the regulation of gene expression.