Type I vs type II FLT3 inhibitors. Gilteritinib is a type I inhibitor. As such, it is more or less an ATP mimetic, and its binding is relatively less influenced by the conformation of the activation loop. “DFG” refers to 3 highly conserved amino acid residues (aspartate-phenylalanine-glycine) at the start of the activation loop. A type II inhibitor fits into a hydrophobic groove adjacent to the ATP binding site, and, in the case of quizartinib, actually fits in between phenyl rings of phenylalanine 691 (F691, the “gatekeeper”) and phenylalanine 830 (F830). Professional illustration by Somersault18:24.