Braas and colleagues performed mass-spectrometry–based metabolomics to assess alternative nutrient uptake in liposarcoma and observed nucleoside consumption and elevated activity of the nucleoside salvage pathway enzyme deoxycytidine kinase (dCK) in patient-derived liposarcoma cell lines and a subset of primary liposarcoma samples. Nucleoside salvage pathway activity could be imaged in vivo by positron emission tomography (PET) using a cytidine-derived tracer, 1-(2′-deoxy-2′-[18F]fluoroarabinofuranosyl) cytosine (FAC), and enhanced the sensitivity of liposarcoma cell lines and xenograft tumors to gemcitabine, a nucleoside analogue prodrug, in a dCK-dependent manner. These results suggest that FAC–PET may identify patients with liposarcoma who will benefit from gemcitabine treatment. For details, please see the article by Braas and colleagues on page 1109.