It is thus possible that the role of p16INK4a in skeletogenesis or body size can be compensated for by other CKIs. Mice deficient in p15INK4b (p15INK4b−/− mice) were born at the expected Mendelian ratios, were fertile and did not exhibit gross morphological or behavioral abnormalities [49]. As in the case of p16INK4a, it is possible that the role of p15INK4b in skeletogenesis or body size can be compensated for by other CKIs. For p18INK4c, wild-type, heterozygous, and null mice appeared indistinguishable

at birth. However, within 2–3 weeks, the p18INK4c−/− Torin 1 molecular weight mice became distinctly larger than their wild-type littermates. By the end of 1 month, the p18INK4c−/−mice were 35–45% larger than their p18INK4c+/+ littermates. The body weights of the p18INK4c−/− mice were increased by 20%, 40%, and 30% at 1, 2, and 3 months, respectively. There was no

obvious difference in the levels of p18INK4c protein between p18INK4c+/+ and p18INK4c+/− tissues, and there was no significant difference in the body and organ size between the wild-type and heterozygous mice. These results suggested that there Caspase inhibitor is no gene dose-dependence for the p18INK4c protein expression [50]. Latres and co-workers [49] also reported that p18INK4c−/− mice were larger than their wild-type littermates. However, the mice generated in their laboratory showed only 20% weight increases at most compared to the wild-type mice. They attributed the quantitative differences seen in the two groups to the different genetic backgrounds of the mice [49]. It is true that p18INK4c was shown to be larger, but the mechanisms involved are not yet fully understood, and extensive studies are needed to elucidate the roles of p18INK4c in skeletogenesis and body size. In a study by Zindy et al. [51], p19INK4d-deficient mice were born at a normal Mendelian ratio, developed into adulthood, had a normal life span.

Except for abnormalities in testicular size and male germ-cell L-gulonolactone oxidase maturation, no other obvious developmental anomalies were observed in the p19INK4d-deficient animals. They did not spontaneously develop tumors [51]. Here again, as in the case of p16INK4a and p15INK4b, it is possible that the role of p19INK4d in skeletogenesis or body size can be compensated for by other CKIs. In conclusion, regarding the INK4 family, these findings raise the hypothesis that p18INK4c has the most important role in skeletogenesis and/or body size. Because Rb−/− embryos which were generated by a conventional knockout strategy died by the 16th embryonic day, Rb was shown to be essential for normal mouse development [52]. In contrast, the Rb+/− mice were developmentally normal except for a pituitary tumor predisposition with nearly complete penetrance [52]. In an in vitro study, it was demonstrated that Rb acts as a transcriptional coactivator of Runx2, which is a master regulator of osteogenic differentiation [53]. Further studies may elucidate the role of Rb in skeletogenesis.