Expression of the human neutrophil elastase (NE) gene is limited to the early stage of myeloid cell differentiation in bone marrow cells. While NE gene expression is controlled mainly at the transcriptional level during bone marrow cell differentiation, the mechanism of transcriptional control is not fully understood. One motif of interest in the 5' flanking region of the gene is the six tandem repeats of a 53-bp nucleotide sequence (REP53) containing a potential binding site for a basic helix-loop-helix protein located at -1032 to -716. The REP53 sequence can function as a non-cell specific transcriptional enhancer which is capable of augmenting heterologous promoter activity. When the single REP53 element was inserted into the pAZ1037 chloramphenicol acetyltransferase (CAT) expression vector immediately upstream of the chicken beta-actin promoter in either normal or inverted orientation and used to transfect K-562 erythroleukemia or HeLa cervical carcinoma cells, these modified vectors achieved 2 to 3-fold higher CAT activity than the parental pAZ1037 vector irrespective of orientation of the REP53.