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.