The upstream stimulatory factor, USF, is a ubiquitously expressed cellular transcription factor that binds to a symmetrical DNA sequence that is found in a variety of viral and cellular promoters. A full-length cDNA encoding the 43-kDa USF protein has previously been isolated. USF contains both helix-loop-helix and leucine repeats, which are involved in regulating its DNA binding and dimerization properties. We report the use of mouse-human somatic cell hybrids and in situ hybridization in localizing the gene-encoding USF to human chromosome 1q22-q23.