CDKN1B and osteosarcoma: Moreover, Runx2 induced G1 cell-cycle arrest through the induction of Cdkn1b (p27KIP1) in osteosarcoma cells, the expression of Cdkn1a (p21CIP1) and Cdkn2a (p19ARF) was reduced in Runx2–/– osteoblast progenitors in vitro, and the introduction of Runx2 induced the expression of Cdkn1b, Cdkn1a, and Cdkn2a, suggesting that they are targeting genes responsible for the inhibitory function of Runx2 in the proliferation of osteoblast lineage cells [36,37].