We also found that RET mRNA levels in patient’s MTC were significantly higher than those found in seven other MTCs with various amino-acid substitutions at position 634 in RET. Subsequent experiments demonstrated that the presence of the synonymous mutation created new exonic splicing enhancer motifs in the mutant allele, which led to more efficient maturation of its transcript and increased expression of a constitutively active RET receptor. The gene discussed is RET; the disease is medullary thyroid gland carcinoma.