Schematic of the FGFR1 mutations at protein level. SP, signal peptide; D1-D3, immunoglobulin-like domains; TM, transmembrane domain; JM, juxtamembrane domain; TK1-2, tyrosine kinase domain (contains two subdomains). The G48S mutation is located in the first immunoglobulin-like domain (D1), involved in the receptor autoinhibition. The R209H mutation is located in D2, responsible for ligand binding and specificity. The E670A mutation lies within TK2, responsible for activating the MAP (mitogen-activated protein) kinase pathway. The nonsense mutations, W4X and R609X, and the frameshift mutations (E84GfsX26, K321RfsX13, S436YfsX3), all lead to premature stop codons.