A genetic alteration (mutation) in the FGFR2 gene causes Crouzon syndrome. The FGFR2 gene gives your body instructions to make a protein called fibroblast growth factor receptor. This protein triggers immature cells to become bone cells during your baby’s development. When there’s a mutation in the FGFR2 gene, the FGFR2 protein is overactive and triggers the immature cells too fast. This causes your baby’s skull bones to fuse prematurely.