Fig. 1.

Reciprocal-labeling scheme. (Top) Mice (4 Min and 4 wild type) were raised on Spirulina-based food. Each mouse was paired with a sibling bearing the opposite genotype. In each pair, 1 mouse was raised on the ¹⁴N diet, and the other was raised on the ¹⁵N diet. (Bottom) The isotopic label was exchanged across genotypes from 1 mouse pair to the next, allowing genotype-dependent differences to be distinguished from various artifacts based on the pattern of ratio inversions across the mouse pairs.