“…It should be noted that the mutation present in the INK4a/ARF locus, deletion of exons 2 and 3, eliminates the function of p16 INK4a (Serrano et al, 1996), but its e ects on the function of p19 ARF have not been unambiguously determined (see Palmero et al, 1998). As previously reported, wild-type MEFs formed colonies very ine ciently compared to p53 7/7 MEFs, INK4a/ ARF D2,3 MEFs, or the immortal cell line of murine ®broblasts NIH3T3 (Harvey et al, 1993;Jones et al, 1996;Serrano et al, 1996) (see Table 1). Interestingly, the colony-formation e ciency of p21 7/7 MEFs was very similar to that of wt MEFs (see Table 1), suggesting that p21 7/7 cells have not acquired the same proliferative advantages as p53 7/7 or INK4a/ ARF D2,3 MEFs.…”