An Arg-Gly-Asp (RGD) peptide that acts on a number of integrins and has a receptor close to the T4 receptor on αvβ3, also blocked the action of T4 on thyroid cancer cells, but a control Arg-Gly-Glu (RGE) peptide did not affect the action of T4. This evidence concerns the gene CD4 and thyroid gland carcinoma.