Prader-willi syndrome is a genetic disorder caused by loss of a portion of a copy of chromosome 15. Common features include early problems with muscle weakness and feeding followed by occult weight gain without an increase in food consumption beginning during late infancy/early toddler period prior to the onset of hyperphagia. Recent research has shown a role for changes in the balance of gut bacteria in weight gain (human and animal), and hyperphagia (animal) studies. The altered gut bacteria balance allows more absorption of calories with increased weight gain on fixed oral calories. We hypothesize that individuals with Prader-willi syndrome have an altered balance in gut bacteria which may influence weight gain and appetite. We propose a pilot study of the gut microbiome among individuals with Prader-willi syndrome focusing on twins (identical twins and fraternal twins) in addition to prospective collection of stool specimens focused upon infants/young children with Prader-willi syndrome moving through age associated dietary changes to assess whether alterations in the gut microbiome may contribute to weight discrepancies and reported differences in hyperphagic tendencies reported by families. If there is alteration in gut bacteria found in PWS, interventions which can change the balance of gut bacteria may be beneficial.