In type B cystinuria mice (Slc7a9-/-Slc3a1+/+) lithiasic phenotype increases from 43% of calculi formation at 2-months of age in the second filial generation to ~85% in the sixth filial generation obtained by intercrossing lithiasic mice in pure C57Bl/6 background (L.Feliubadaló, M.Palacín and V.Nunes; unpublished results). The gene discussed is SLC3A1; the disease is cystinuria type B.