D, Nitric oxide synthase (NOS) activity, an enzyme involved in arginine metabolism, in healthy (n=3; blue) and in residual (n=3; green) mouse mammary glands. The difference is statistically significant by unpaired two-samples t-test. E, Fractional labeling of lactate after cultivation of isolated regressed (n=3; green) and healthy (n=3; blue) mouse mammary glands in cell growth medium supplemented with [U-13C] glucose for 8 h (Methods). The three-carbon labeled (13C) isotopologue (M+3) of lactate is depicted and shows enrichment in the residual cells of the regressed mammary glands. The difference is statistically significant by unpaired two-samples t-test. Data information: Numbers of replicates correspond to individual mice or humans. Numbers marking comparisons (gray lines) show p values (corresponding statistical tests are described in individual panel legends). 