ACVRL1 and hematocrit: Endoglin (ENG) mutations cause what is referred to as HHT1 (OMIM #187300) [14], activin receptor-like kinase 1 (ACVRL1) mutations cause HHT2 (OMIM #600376) [15], while mothers against decapentaplegic homolog 4 (MADH4 or SMAD4) mutations cause a syndrome which combines familial juvenile polyposis and HHT (JP/HT; OMIM #175050) [16].