FUCA1 and fucosidosis: Although the genotype-phenotype correlation is still unclear in fucosidosis (Willems et al., 1991, 1999, 1988; Stepien et al., 2020; Beratis et al., 1977), it is suggested that patients with the severe and fast-progressing form typically exhibit no FUCA1 activity (Willems et al., 1988; Stepien et al., 2020).