CDKN1A and acute lymphoblastic leukemia: Moreover, genetic ablation of Xbp1 caused cell cycle arrest in the G1 phase and was accompanied by an upregulation of cyclin‐dependent kinase inhibitors p21CIP1/WAF1 and p27KIP1 (Figure 1E), implicating XBP1 as critical for the survival and proliferation of the pre‐B NRASG12D ALL subset.