In each disease, this level is different: in CF, 30–35% of normal CFTR activity is sufficient; in DMD – 20-30% of the full-length dystrophin (but according to others, even 1% is sufficient); in MPS I-H, even 0.4–1% of the α-L-iduronidase level is enough to alleviate disease symptoms (Hoffman et al., 1988; Keeling et al., 2014). Here, CFTR is linked to cystic fibrosis.