Here, the gene expression analysis revealed a significant up-regulation of genes encoding cell cycle proteins, including CDKs, cyclins and endogenous cyclin-dependent kinase inhibitors (CKI), in malignant mouse MTC, suggesting that p25-CDK5-dependent MTC tumorigenesis is associated with alterations in cell cycle regulation. This evidence concerns the gene CDK5R1 and medullary thyroid gland carcinoma.