Notably, the pattern of enzymatic alteration differs between monoallelic and biallelic mutations depending on their genetic and functional characteristics, as exemplified by G6PC, which requires biallelic mutations to affect its activity (Lei et al., 1993), and single FGFR3 mutations that cause achondroplasia through abnormal enzymatic activity (Rousseau et al., 1994). Here, FGFR3 is linked to achondroplasia.