Provided herein are methods of treating beta-thalassemia in a subject comprising administering to the subject an activin type II receptor (ActRII) signaling inhibitor (e.g., an activin ligand trap) and utilizing one or more in vitro cell culture methods provided herein in (i) selection of the subject to be treated according to the methods provided herein； and/or (ii) monitoring of the subject being treated according to the methods provided herein.
