The authors would like to thank Frank Kist, Jodi Dagget, Edward Mallick, Brian Sloat, and Judith Rodda for expert technical assistance, and Dr. Susan Hutson for the gift of E2 antiserum. This work was supported by National Institutes of Health Grants DK51960, DK57386, DK57956, AA10422, the Scott C. Foster Metabolic Disease Fund, and the MSUD Family Support Group.