A general term for a group of about fifty rare METABOLIC DISORDERS in which there is a chromosomal abnormality resulting in the absence of an enzyme, thus allowing build-up of metabolites which should have been processed safely by that enzyme. Amongst these conditions are BATTEN'S DISEASE, GAUCHER'S DISEASE and. Prenatal diagnosis is usually available in families known to be at risk and some of the conditions can be treated, for example by BONE MARROW TRANSPLANT in certain of the and in Gaucher's and Fabry's diseases.