Progression Associated Protein (PAP), a new transmembrane receptor encoded by 157 amino acids was identified as a progression marker by differential display techniques of mammary carcinoma cell lines with different metastatic properties. PAP has been isolated independently by several groups in humans, mouse and rabbit, and has received different designations (TMP, EMP-1, CL-20 and B4B). As a first step towards understanding the biological function of PAP we have investigated prevalence and regulation of expression of PAP in human tissues, primary cells and tumor cell lines. PAP is expressed in most, but not all human tissues and investigation of several human mammary carcinoma cell lines with different metastatic characteristics revealed a correlation between expression of PAP and their invasive and metastatic properties. Expression of PAP under proliferation and growth-arrest conditions was investigated by a combination of Northern blot and FACS analysis. Expression of PAP was associated with proliferative status of the cells and its down-regulation was linked to induction of G1 arrest. These data indicate that PAP is inversely regulated compared to PMP22, a growth-arrest gene which belongs to the same gene family as PAP.