P21, a cyclin-dependent kinase (CDK) inhibitor, is downregulated in a variety of cancer types,33 and plays critical roles in multiple cellular processes during unperturbed cell growth by directly binding to kinases related to G1/S transition, such as CyclinD/CDK4, CyclinD/CDK6 and CyclinE/CDK2.34, 35, 36 Here, we found P21 was a downstream regulator involved in SNHG20-mediated growth arrest in lung cancer cells. Here, CDK4 is linked to lung cancer.