There are different potential routes by which 13C11-tryptophan-derived carbons could be metabolized into serine and purines within tumors in vivo: (1) tumor dependent (i.e., primarily via tumor-specific IDO1 expression), (2) systemic metabolism dependent (i.e., predominantly via IDO1 expressed in healthy tissues, which provides labeled formate or serine into the circulation, taken up by tumors), or (3) a combination of these two routes. The gene discussed is IDO1; the disease is neoplasm.