In this knockout strain, a neo cassette replaces exon 2 of the of the cyclin-dependent kinase inhibitor 1A (Cdkn1a or p21CIP1/WAF1) gene. Homozygotes are viable, fertile, and normal in size. P21CIP1/WAF1 is expressed, in a p53-dependent manner, during embryogenesis and postnatal development primarily in dividing cells. Although, actively dividing cells in these mice do not arrest in G1 phase of the cell cycle in response to DNA damage, these mice do not develop tumors as mice deficient in p53 do. These mice may be useful for studying cell cycle timing and arrest during cellular proliferation. The Jackson Laboratory 2024, used with permission.