methods for screening for a defective allele of a cofactor-administered enzyme-coding gene, for detecting a predisposition to a cofactor-dependent enzyme deficiency, for identifying and / or characterizing an enzyme deficiency within a metabolic pathway in a patient , to treat a metabolic enzyme deficiency in a patient, to screen for the risk of a condition or disease associated with aberrant foliate / homocysteine metabolism, to screen for potential chemotherapeutic response, and to screen for chemotherapeutic toxicity, to evaluate remediable enzyme deficiencies in a metabolic pathway, isolated nucleic acid, arrangement to detect a defective allele of a gene in the foliate / homocysteine metabolic pathway