Group A proteins were identified as defective ABCA12 (ATP-binding cassette sub-family A member 12), which causes ARCI4B (autosomal recessive congenital ichthyosis 4B) and is associated with reduced protease regulation and skin-barrier dysfunction [43,44,45,46], and defective ABCC2 (ATP-binding cassette subfamily 2), which causes DJS (Dubin–Johnson syndrome) and is associated with hyperbilirubinemia [47]. Here, ABCC2 is linked to Harlequin ichthyosis.