A product of the p16 tumor suppressor gene (genes, p16). It is also called INK4 or INK4A because it is the prototype member of the INK4 CYCLIN-DEPENDENT KINASE INHIBITORS. This protein is produced from the alpha mRNA transcript of the p16 gene. The other gene product, produced from the alternatively spliced beta transcript, is tumor suppressor protein p14arf. Both p16 gene products have tumor suppressor functions.