The p16Ink4/MTS1/CDKN2 is a cell-cycle regulatory inhibitor of cyclin-dependent kinase 4 (cdk4), and a candidate tumour suppressor whose gene on chromosome band 9p21 is frequently deleted or mutated in diverse types of cancer. Cdk4 in association with its D-type cyclin partners, together with p16Ink4, and the product of the retinoblastoma tumour-suppressor gene (pRB), appear to constitute a G1-phase-controlling pathway which can become de-regulated through oncogenic aberrations of any of the components. In an attempt to elucidate the underlying molecular mechanisms, we have now surveyed expression of p16Ink4, at the protein and the mRNA levels, in 21 human cell types expressing normal pRB, as compared with another series of 21 cell lines whose pRB is mutant and/or inactivated through sequestration by DNA tumour virus onco-proteins. In contrast to aberrant lack of p16 expression in the majority of RB-positive cell types, expression of apparently normal (as shown by electrophoretic mobility and/or the ability to form protein-protein complexes with cdk4 in vivo) p16 was uniformly preserved in the cancer cell lines whose RB function was compromised. These data indicate that p16 operates upstream of pRB along the same pathway in G1. The results are discussed in view of the nature of a selective growth advantage potentially gained by cells through de-regulation of this key cell-cycle control mechanism.