The purpose of this study is to evaluate the effectiveness of magnesium oxide supplements on the reversal of calcium deposits in the skin, and the yellow bumps and folds of skin in subjects with pseudoxanthoma elasticum (PXE). Magnesium oxide is a dietary supplement that has been shown in some research to reduce these calcium deposits. This study consists of two parts. The first part is a year-long, double-blind, placebo-controlled study. Part two is an open-label, year-long study. In Part 1, qualified subjects will be randomized to receive either magnesium oxide supplements or placebo, in a 1:1 ratio for the first 12 months. The starting dose will be 1000 mg daily, and depending on tolerability, doses may be decreased. Baseline evaluations will be comprised of: blood tests; clinical evaluations; skin biopsy; eye examination; bone density test; and photography of skin lesions. Subjects will be evaluated at week 2, week 6, month 3, and then every 3 months during the first year. Upon completion of the first year, barring any safety concerns, all subjects will be administered magnesium oxide supplements for up to one additional year. Subjects will undergo the same evaluations/ procedures every 3 months. We hypothesize that the magnesium oxide will cause a reduction in calcifications in the subject's soft tissue/skin. Funding Source - FDA OOPD.