Ribose-5-phosphate isomerase deficiency

Ribose-5-phosphate isomerase deficiency, abbreviated RPID, is an extremely rare human disorder caused by mutations in ribose-5-phosphate isomerase, an enzyme of the pentose phosphate pathway. Only four cases are known, all diagnosed between 1984 and 2019. On that count the source ranks it as the second rarest disease known, after Fields condition, which affects three known individuals including Catherine and Kirstie Fields. The very small number of patients means that what is known about the disorder rests on a handful of individual case reports rather than on population studies.