GPC4 and skeletal dysplasia: Defects in glypicans, a group of heparan sulfate proteoglycans (HSPGs), can cause abnormal skull and skeletal dysplasia in both humans (Simpson–Golabi–Behmel syndrome: glypican 3, GPC3; Keipert syndrome: glypican 4, GPC4) and zebrafish (Knypek: Gpc4) (LeClair et al., 2009; Tenorio et al., 2014).