The availability of Hertwig’s anemia (an) mutant mice (Cdk5rap2an/an) that carry exon 4 inversions in the Cdk5rap2 gene, allowed us to test whether such Cdk5rap2an/an mutation affects body weight and size, and whether the senescence-associated phenotypes observed in CDK5RAP2-deficient BJ cells are also exhibited in Cdk5rap2an/an embryos and MEFs. The gene discussed is CDK5RAP2; the disease is anemia.