 P15INK4b has a different reading frame that is physically separated from p16INK4a and ARF.P16INK4a and ARF have different first exons that are spliced to the same second and third exon.While those second and third exons are shared by p16INK4a and ARF, the proteins are encoded in different reading frames meaning that p16INK4a and ARF are not isoforms, nor do they share any amino acid homology.