ATP-NAD kinase phosphorylates NAD to produce NADP by using ATP, whereas ATP-NADH kinase phosphorylates both NAD and NADH. Three NAD kinase homologues, namely, ATP-NAD kinase (Utr1p), ATP-NADH kinase (Pos5p) and function-unknown Yel041wp (Yef1p), are found in the yeastSaccharomyces cerevisiae. In this study, Yef1p was identified as an ATP-NADH kinase. The ATP-NADH kinase activity of Utr1p was also confirmed.