5'-S-(3-Amino-3-carboxypropyl)-2',3'-O-(1-methylethylidene)-5'-thio Adenosine-D4 is an intermediate in the synthesis of S-(5'-Adenosyl)-L-homocysteine-d4 (A291501). It is a labeled S-(5'-Adenosyl)-L-homocysteine (A291500), a novel biomarker for predicting susceptibility of a subject to a mental or neurodegenerative disorder.