Study of Safety & PK of Luspatercept (ACE-536) in Pediatric Participants With Beta (β)-Thalassemia

Inclusion Criteria * Participants must be 6 years to \< 18 years of age at the time of signing the informed consent form (ICF)/informed assent form (IAF). * Participants (and when applicable, parent/legal representative) must understand and voluntarily sign an ICF/IAF prior to conducting any study-related assessments/procedures. * Participants (and when applicable, parent/legal representative) is willing and able to adhere to the study visit schedule and other protocol requirements. * Participants must have documented diagnosis of β-thalassemia or Hemoglobin E/β-thalassemia. * Transfusion dependence (TD): 1. TD participant i. Participant is regularly transfused, defined as: ≥ 4 RBC transfusion events in the 24 weeks prior to enrollment with no transfusion-free period ≥ 42 days during that period. Note: For the purpose of the study, transfusions administered over 2 or 3 consecutive days are considered as part of a single transfusion event. Participant must have a history of regular transfusions for at least 2 years. 2. NTD participant (ex-US sites only) i. Participant must have received \< 4 RBC transfusion events in the 24 weeks prior to enrollment. ii. Participant must not be on a regular transfusion program and must be RBC transfusion-free for at least 8 weeks prior to enrollment. iii. Participant must have mean baseline hemoglobin ≤ 10 g/dL, based on a minimum of 2 measurements ≥ 1 week apart within the 12-week screening period; at least 1 measurement should be collected within 4 weeks prior to enrollment; hemoglobin values within 21 days post-transfusion will be excluded. * Participants have Karnofsky (age ≥16 years) or Lansky (age \< 16 years) performance status score ≥ 50 at screening. * Female children of childbearing potential (FCCBP), individuals of childbearing potential (IOCBP), and male (as assigned at birth) participants that have reached puberty (and when applicable, parent/legal representative) must agree to undergo physician-approved reproductive education and discuss the side effects of the study therapy on reproduction. * Female children of childbearing potential, defined as females who have achieved menarche with or without breast development in Tanner Stage 2 or greater and have not undergone a hysterectomy or bilateral oophorectomy and IOCBP defined as a sexually mature woman who has achieved menarche at some point, has not undergone a hysterectomy or bilateral oophorectomy and has not been naturally postmenopausal for at least 24 consecutive months (ie, has had menses at any time in the preceding 24 consecutive months) must meet the following conditions below (Note: Secondary amenorrhea from any cause does not rule out childbearing potential. Breast development at Tanner Stage ≥2 alone does not reliably indicate reproductive potential): * Medically supervised serum pregnancy tests with a sensitivity of at least 25 mIU/mL must be conducted in Female children of childbearing potential (FCCBP)/ individuals of childbearing potential (IOCBP), including those who commit to complete abstinence. Female children of childbearing potential/ individuals of childbearing potential (IOCBP) must have 2 negative pregnancy tests as verified by the Investigator prior to starting study therapy (one of these tests should be performed by central laboratory). Female children of childbearing potential/ individuals of childbearing potential (IOCBP) must agree to ongoing pregnancy testing during the course of the study at the End of Treatment (EOT) visit and at the 9-week Safety Follow-up visit. * Female participants must, as appropriate to age and at the discretion of the site Investigator, either commit to true abstinence\* from heterosexual contact (which must be reviewed on a monthly basis) or agree to use, and be able to comply with, effective\*\* contraception without interruption, 28 days prior to starting IP, during the study therapy (including dose interruptions), and for 12 weeks (approximately 5 times the mean terminal t1/2 of luspatercept based on multiple-dose PK data) after discontinuation of study therapy. * Male (as assigned at birth) participants, as appropriate to age and the discretion of the study physician: * Must practice true abstinence\* (which must be reviewed on a monthly basis) or agree to use a synthetic or latex condom during sexual contact with a pregnant female or a Female children of childbearing potential (FCCBP)/ IOCBP while participating in the study, during dose interruptions and for at least 12 weeks (approximately 5 times the mean terminal t1/2 of luspatercept based on multiple-dose PK data) following IP discontinuation, even if he has undergone a successful vasectomy. * True abstinence is acceptable when this is in line with the preferred and usual lifestyle of the participant. \[Periodic abstinence (eg, calendar, ovulation, symptothermal, post-ovulation methods) and withdrawal are not acceptable methods of contraception.\] \*\* Agreement to use highly effective methods of contraception that alone or in combination result in a failure rate of a Pearl index of less than 1% per year when used consistently and correctly throughout the course of the study. Such methods include: Combined (estrogen and progesterone/progestin containing) hormonal contraception: Oral; Intravaginal; Transdermal; Progestogen/progestin only hormonal contraception associated with inhibition of ovulation: Oral; Injectable hormonal contraception; Implantable hormonal contraception; Placement of an intrauterine device (IUD); Placement of an intrauterine hormone-releasing system (IUS); Bilateral tubal occlusion; Vasectomized partner; Sexual Abstinence. Exclusion Criteria * Participant has a diagnosis of Hemoglobin S/β-thalassemia or alpha (α)-thalassemia (eg, Hemoglobin H); β-thalassemia combined with α-thalassemia is allowed. * Participant has of active hepatitis C (HCV) infection, as demonstrated by a positive HCF-ribonucleic acid (RNS) test of sufficient sensitivity