Human complement component C4 is encoded by two structurally distinct loci in the major histocompatibility complex (MHC) class III region. The two isotypes, C4A and C4B, differ at only four residues in the C4d fragment, but C4 constitutes the most polymorphic of the complement components. It is not known, however, whether the regions involved in the regulation of C4 expression also display polymorphic variation. By using the technique of DNase I hypersensitivity mapping, we established that the only area of transcriptional activity for C4 in the hepatocyte cell line, HepG2, occurs approximately 500 base pairs upstream of the transcriptional start site. This region was found to be remarkably constant in sequence when analyzed in the context of differing MHC haplotypes including HLA B57, C4A6, C4B1, DR7, which has been correlated with reduced expression of the C4A isotype. Similarly, polymerase chain reaction followed by single-strand conformation polymorphism analysis failed to demonstrate any promoter polymorphisms in 103 individuals comprising 52 systemic lupus erythematosus patients and 51 healthy controls.