C4A and Costello syndrome: Numerous studies link CHST11 to fibroblasts: primary fibroblasts from Costello syndrome patients show reduced chondroitin-4-sulfate (C4S) and lower CHST11 mRNA/protein expression; oncogenic HRAS expression in normal fibroblasts inhibits CHST11, while interfering with oncogenic HRAS signaling in Costello syndrome fibroblasts upregulates CHST11 (55).