Structural model of ALK1 cytoplasmic domain. A structural model of ALK1 intracellular domain was generated by homology modeling to the corresponding domain of ALK5 deduced from its crystal structure.^37,38  The different domains of the proteins are indicated in different colors: GS box in green (amino acids 172-201), phosphate binding domain in yellow (amino acids 208-214), Smad binding loop (L45) in magenta (amino acids 263-270), catalytic loop in orange (amino acids 328-348), activation domain in cyan (amino acids 351-375), and NANDOR box in blue (amino acids 479-489). The positions of the 18 missense mutations, overlaid on the modelized structure, are indicated in red for the pathogenic mutations and in purple for the rare polymorphisms.