In other cases, however, a possible avian compensatory mechanism and/or functional impact for the avian gene loss is unknown, including cases of severe disease or lethal phenotypes when the genes are deleted in other organisms, such as ABCD1 and PRX (central and peripheral demyelinating diseases), FGD1 (affecting bone growth), and FTSJ1 and SYP (X-liked mental retardation). The gene discussed is FGD1; the disease is demyelinating disease.