The invention features polypeptides that include an extracellular ActRlla variant. In some embodiments, a polypeptide of the invention includes an extracellular ActRlla variant fused to an Fc domain monomer or moiety. The invention also features pharmaceutical compositions and methods of using the polypeptides to treat diseases and conditions involving weakness and atrophy of muscles, e.g., Duchenne muscular dystrophy, facioscapulohumeral muscular dystrophy, inclusion body myositis, amyotrophic lateral sclerosis, sarcopenia； or cancer cachexia； or metabolic diseases, e.g., obesity, Type-1 diabetes, or Type-2 diabetes.
