AGXT and Krabbe disease: Three pieces of information help establish the foundation for this hypothesis: (1) preliminary findings indicate that D-cycloserine prolongs the life of twitcher mice, (2) D-cycloserine inhibits SPT (51, 57), which is the rate-limiting step for sphingolipid production (49, 50), and (3) reductions of SPT activity result in reduced production of galactosylceramide and psychosine, which are the substrates for the deficient enzyme galactosylceramidase in Krabbe disease (103).