Frendo et al. (1998a, 1994) observed that in MTC and TT cell lines, the shorter isoform of CTR (CTR C1a) was expressed and that CTR mRNA was present both in normal and tumor tissue with higher levels in tumors and without differences among the different clinical forms of MTC (Frendo et al., 1998b). This evidence concerns the gene CALCR and medullary thyroid gland carcinoma.