The CD9 antigen, initially discovered on B lineage leukemic cells, belongs to the tetraspan superfamily of surface molecules. If no precise function has been assigned to any of these molecules, there are some indications that they could be involved in cell adhesion and cell migration, as well as malignant progression. The CD9 antigen is associated with surface proteins such as VLA integrins or HB-EGF precursor. Transfection of CD9 in melanoma cells reduces tumor growth and metastasis. The heterogenous distribution of the CD9 antigen suggests a complex regulation of its expression. We have previously characterized the CD9 gene and shown that transcription could be initiated at several sites in the TATA-less 5'-flanking region. We show here, using as a model two human leukemic cell lines with erythromegakaryocytic potential, HEL and K562, that the [-205, -154] region supports a promoter activity when cloned ahead of a CAT reporter gene. Mutagenesis analysis suggested the presence of a positive element located within the [-170, -154] region. Gel shift experiments using HEL extracts were compatible with the binding of the transcriptional factor Sp1 to the [-237, -205] region and indicated that a non-identified protein binds to the 3' end of the [-205, -154] region.