As the PNP-mediated new salvage pathway is also present in mammals (see Additional file 2: Table S2 and Additional file 3: Text S2), it remains to be tested whether human PNP (counterpart of xapA) is also able to utilize NAM to synthesize NR as an alternative to pathway II (i.e., via pathway IIIb), thus responsible for the slow anti-cancer action of FK866. This evidence concerns the gene PNP and cancer.