The present invention provides Shiga toxin A Subunit derived polypeptides, scaffolds, and cell-targeting molecules comprising amino acid substitutions which equip the molecules with site-specific positions (and often unique amino acid residues in the molecule) for linking other molecules while retaining Shiga toxin function(s), such as, e.g., efficient intracellular routing and/or potent cytotoxicity. The present invention also provides cell-targeting molecules, and/or components thereof, which comprise site-specific positions for linking other molecules, such as, e.g., agents that alters a property of the cell-targeting molecule or a cargo for delivery. Certain molecules comprising a polypeptide of the present invention exhibit reduced immunogenicity and/or are well-tolerated by mammals. The cell-targeting molecules of the present invention, and compositions thereof, have uses, e.g., for the selective delivery of cargos to target-expressing cells and as diagnostic and/or therapeutic molecules for the treatment of a variety of diseases, disorders, and conditions, which include genetic disorders, genetic predispositions, infections, cancers, tumors, growth abnormalities, and/or immune disorders.
