Ectopic expression of ALKALs and overactivation of DrLtk leads to supernumerary iridophores in D. rerio. (A) Domain structure of the human and zebrafish ALK/LTK RTK family. GR, glycine rich; L, LDLa; M, MAM; PTK, kinase domain; TM, transmembrane domain. The activating F993I (DrLtk) and F1174L (HsALK) mutations within the kinase domain are indicated. (B) ltk loss-of-function shady mutants (ltkj9s1) lack iridophores, while gain-of-function moonstone (ltkmne) exhibit increased numbers of iridophores. (C) Alignment of ALKAL proteins from different species. Underlined, the FAM150 domain; red asterisks, conserved Cys. Note high conservation of the C-terminal half of the FAM150 domain.