It is usually caused by infectious agents (e.g., E. coli, classical HUS), but can also be attributed to monogenic diseases which are primarily impacting cobalamin and folate metabolism (atypical HUS, aHUS): Combined methylmalonic acidemia and homocystinuria defects (CblC, CblD, CblF, CblJ) do present with HUS in 10–25% of reported patients (n = 396) [37]. Here, MMADHC is linked to hemolytic-uremic syndrome.