The 5' region of the gene encoding the human 16 kDa DNA-binding and apoptosis-inducing C1D protein was analysed for promoter activity. Sections of this region were cloned into a promoterless vector containing the enhanced green fluorescent protein (EGFP) as reporter gene. Expressed EGFP was estimated in transfected cells by quantitative fluorescence microscopy. The sequence between mRNA positions ATG -868 and ATG -12 results in relatively highest EGFP expression in transiently transfected human and murine cells. The upstream segment immediately adjacent to the 5' end of the most active fragment was identified as an inverted LINE-1 repeat element. Transient transfection experiments point to the presence of cis-acting repressing sequences on this LINE-1 element which reduce the transcriptional activity of the basal C1D promoter in human and murine cells by more than 95%. This result supports previous evidence suggesting that LINE-1 sequences may function as regulatory elements to control the expression of nearby genes.