 In humans, ARF is translated into the 14kDa, 132 amino acid p14ARF protein, and in mice, it is translated into the 19kDa, 169 amino acid p19Arf.The E1β protein segment of mouse and human ARF are 45% identical, with an overall ARF identity of 50%, compared to a 72% identity between mouse and human INK4a E1α segment, and a 65% overall identity.Although the INK4a and ARF proteins are structurally and functionally different, they are both involved in cell cycle progression.