• Male or female with RBC membranopathy or congenital dyserythropoietic anemia type II (CDAII). Diagnosis must be supported genetically by a ACMG class 3 (VUS), 4 or 5 variant.

• Age ≥18 years.

• Average hemoglobin (Hb) concentration (average of at least 2 Hb measurements separated by a minimum of 7 days the during screening period) must be less than 13.0 g/dL for males and 11.0 g/dL for females. Patients with average Hb \>10.0 g/dL for males and females must meet at least one of the following additional criteria:

‣ Splenomegaly (length ≥12.5 cm)

⁃ Fatigue attributed to hemolysis

⁃ Active hemolysis as evaluated by one or more of the following: haptoglobin, bilirubin, LDH, reticulocytes

• Subjects must start or continue taking at least the equivalent of daily 0.8 mg oral folic acid for the duration of the study.

• Have adequate organ function, as defined by:

‣ Serum aspartate aminotransferase (AST) ≤2.5 × ULN (unless the increased AST is assessed by the Investigator as due to hemolysis and/or hepatic iron deposition) and alanine aminotransferase (ALT) ≤2.5 × ULN.

⁃ Normal or elevated levels of serum bilirubin. In subjects with serum bilirubin \> ULN, the elevation must be attributed to hemolysis with or without Gilbert's syndrome and must not be associated with choledocholithiasis, cholecystitis, biliary obstruction, or hepatocellular disease Elevated bilirubin attributed to hemolysis with or without Gilbert's syndrome is not exclusionary.

⁃ Estimated glomerular filtration rate ≥45 mL/min/1.73 m2 by Chronic Kidney Disease Epidemiology Collaboration creatinine

• Be willing and able to give written informed consent and to comply to all study procedures for the duration of the study.

• For women of reproductive potential, have a negative urine or serum pregnancy test during the Screening Period (Day -50 to Day -1). Women of reproductive potential are defined as sexually mature women who have not undergone a hysterectomy, bilateral oophorectomy or tubal occlusion; or who have not been naturally postmenopausal (i.e. who have not menstruated at all for at least the preceding 12 months prior to signing informed consent), or has a known diagnosis of hypogonadotropic hypogonadism.

• For women of reproductive potential, be abstinent as part of their usual lifestyle, or agree to use a highly effective method of contraception. A highly effective form of contraception is defined as combined (estrogen and progestin containing) hormonal contraceptives (oral, intravaginal, or transdermal) associated with inhibition of ovulation; progestin-only hormonal contraceptives (oral, injectable, or implantable) associated with inhibition of ovulation; intrauterine device; intrauterine hormone releasing system; bilateral tube occlusion; or vasectomized partner. Women of reproductive potential using hormonal contraception as a highly effective form of contraception must also utilize an acceptable barrier method while enrolled in the study and for at least 28 days after their last dose of study drug. An acceptable barrier method includes male or female condoms with or without spermicide, and cervical cap, diaphragm, or sponge with spermicide.