The first link between MTHFR and disease was discovered in 1972 when Mudd et al reported several cases of homocystinuria, a presence of homocysteine in the urine, that could not be explained by deficiencies in other enzymes known to cause this condition. Following analysis of enzyme activity levels in the patients’ cell extracts, researchers discovered severe deficiency in MTHFR.  This deficiency leads to a decreased ability to convert homocysteine to methionine, resulting in significantly elevated levels of homocysteine (Mudd et al, 1972).  This discovery made by Mudd and his colleagues was the first report of severe MTHFR deficiency.  Further study of this protein would later reveal several conditions resulting from more mild deficiencies.