Aspects of the disclosure relate to transferrin receptor 1 (TfR1)-binding proteins. In some cases, humanized TfR1-binding proteins are described. Embodiments include methods for treating one or more conditions, for example cancer, using a humanized TfR1-binding protein. In some embodiments, the disclosed methods and compositions involve one or more antibodies that are capable of binding TfR1. Certain aspects relate to humanized antibodies and antibody-like molecules comprising one or more amino acid substitutions (e.g., backmutations). Additional aspects relate to combination treatments with TfR1-binding proteins and one or more additional therapeutics.