Scheme of mutant RET cDNA constructs. (A) RET cDNA encoding a long (1114 amino acids) isoform in which the cysteine at codon 634 was replaced by arginine (C634R; RET-MEN2A) or the methionine at codon 918 was replaced by threonine (M918T; RET-MEN2B). (B) Mutant RET cDNA encoding a long isoform in which the extracellular domain was deleted (RET-PTC-1). In RET-PTC-1, the cysteine at codon 365 or 376 was replaced by alanine (Ret-PTC-1–C365A, Ret-PTC-1–C376A). SS, signal sequence; CAD, cadherin-like domain; CYS, cysteine-rich region; TM, transmembrane domain; TK1, tyrosine kinase domain 1; TK2, tyrosine kinase domain 2; aa, amino acids.