NTRK1 and multiple endocrine neoplasia type 2: Multiple endocrine neoplasia type 2A (MEN2A) or Sipple Syndrome is an autosomal dominant syndrome, first described by Sipple [1] and later characterized in multiple kindreds by Schimke [2], caused by misense mutations in the RET protooncogene [3,4], a tyrosine kinase receptor.