CREB1 and thyroid cancer: However, the amplification of CREB activation has been invoked to explain thyroid hyperplasia in mice with a knock-in for Ccdc6−ex2, a gene found to be rearranged with RET in PTC44, or sustained cellular proliferation due to the CD44 receptor45 or transmembrane serine protease 446, both of which are overexpressed in thyroid cancer cells.