MTHFR and hyperinsulinemic hypoglycemia, familial, 4: Based on (I) the similarities of symptom profiles between MTHFR deficiency and RTT, (II) parallels in MECP2 and MTHFR methylation and (III) the finding that hyperhomocysteinemia over time causes irreversible neurological damage [27], it is useful to explore profiles of rs1801133 and rs1801131 polymorphisms in RTT patients.