Therefore, in CKD, the variations in serum NAD + metabolites were possibly influenced by NNMT since the upstream metabolites of NNMT, such as NAM and NMN (NAD + precursors), were declined, while the downstream metabolites of NNMT, such as N-Me-2PY and N-Me-4PY (final NAD + metabolites), were elevated with CKD progression. The gene discussed is NNMT; the disease is chronic kidney disease.