The luciferase reporter gene system was used to assay the basal and Ara-C induced promoter activity of the human CR1 gene in K-562 erythroleukemia cells. Based on the results from clones of nested deletions, both basal and induced reporter gene activity fell to promoterless levels between constructs containing 79 bp (-79) and 41 bp (-41) upstream of the transcription start site. The -79 fragment was shifted in electrophoretic mobility assays using nuclear extracts from Ara-C induced and non-induced cells while the -41 bp fragment was not shifted. These data suggest that the 38 bp region between these constructs is necessary for the transcriptional activity of the CR1 gene and is involved in specifically binding transcription factors from the nuclei of induced and non-induced cells. Several potential transcription factor binding sites in this region were identified.