FGFR4 and isolated hemihyperplasia: In 1999, Walteret al. mapped a familial form of IH to 5q31-33, housing three candidate genes—fibroblast growth factor receptor-4 (FGFR4), platelet-derived growth factor receptor-β (PDGF-β), and fms-related tyrosine kinase-4 (FLT4)3—and in subsequent work found that a small number of IHs (2 out of 15 studied cases) harbor somatic mutations in vascular endothelial growth factor (VEGF)-receptor 2 (p.P1147S) andVEGFR3 (p.P954S) (also known asFLT4)4.