All mouse models showing combined methylmalonic aciduria and homocystinuria (Mmachc, Lmbrd1) and mouse models with isolated homocystinuria (Mtr, Mtrr) are lethal at early embryonic stages, suggesting that elevated homocysteine levels and/or reduced methionine formation cause early embryonic lethality. The gene discussed is MTRR; the disease is homocystinuria.