We used the yeast qns1 mutant to screen for natural sources of nicotinamide riboside and, as shown in Figure 5, we found it in a vitamin fraction of cow's milk. Unlike the original screen for vitamins in protein-depleted extracts of liver for reversal of black tongue in starving dogs (Elvehjem et al., 1938), this assay is pathway-specific in identifying NAD+ precursors. As shown in Figure 1, Figure 2, because of the qns1 deletion, nicotinic acid and nicotinamide do not score positively in this assay. Because the factor from milk requires nicotinamide riboside kinase for growth, the nutrient is clearly nicotinamide riboside and not NMN or NAD+.