Despite genetic, pharmacological and kinetic evidence validated by a quantitative assay showing that nicotinamide mononucleotide (NMN) is dephosphorylated to nicotinamide riboside before cellular internalization1 , solute carrier family 12 member 8 (Slc12a8), which is widely expressed and annotated as a Na+/K+ Cl− transporter, has been nominated to be an NMN transporter2 . The analytical methods, transport data and interpretation underlying this assignment are not sound and do not support transport of NMN by Slc12a8.