MMAA and optic nerve disorder: In the cblA patient, the c.439+4_439+7del p (?)variant inherited from the mother was predicted to delete the canonical splice site of exon 2.29 Being at risk, before P‐OHCbl‐DI, to develop later mild optic neuropathy and/or CRD, because of previous uMMA levels above the approximate threshold of 2000 μmol/mmol/creatinine,12, 13 the cblA patient 4 had normal visual and renal function on early follow up.