In tissues lacking serine dehydratase and serine aminotransferase, such as skeletal muscle where the rat sarcoma model was assayed, preferential reorientation for nucleotide formation cannot be achieved by deletion of nonexistent competing enzymes and must instead rely only on substantially increased activity of serine hydroxymethyltransferase in tumors [17, 20]. The gene discussed is SDS; the disease is sarcoma.