The promoter of the human BCR gene, regulating the transcription of the chimeric BCR/ABL mRNA in leukemia, has been isolated and characterized. A region of 1.1 kb immediately 5′ to the transcription start site was analyzed in detail by sequencing, DNasel footprinting, gel retardation and functional studies. These experiments localized a minimal promoter to a 650 bp sequence, composed of 270 bp of 5′ flanking sequences and 380 bp of exon 1 transcribed sequences. The promoter region includes a TTTAA box, one Sp1 site and a novel protein-binding sequence absolutely necessary for efficient transcription in vivo . Six additional protein-binding regions were identified more to the 5′. Of these, one is found in an inverted repeat in the 3′ coding and splice donor region of BCR exon 1.