P35, a critical regulator of CDK5 activity, is a short-lived protein and its expression is finely regulated through ubiquitin-proteasome degradation26 or through the presence of regulatory elements affecting its transcript stability.27 We observed that, while p35 is virtually absent in healthy controls, it results to be upregulated in cancer cells at protein but not at RNA level. This evidence concerns the gene CDK5R1 and cancer.