Plasma tHcy levels are elevated with CBS deficiency. Plasma levels of (A) tHcy and (B) methionine were measured in 20-week-old mice (n = 20-30 mice per group). Values are mean ± SEM; **P < .001 compared with Cbs^+/+ and Tg-I278T Cbs^+/+ mice and *P < .05 compared with Tg-I278T Cbs^+/+ mice.