A method for identifying an amino acid sequence of a binding region for use in constructing one or more chimeric cytotoxic protein, wherein the one or more chimeric cytotoxic protein comprises: i) a ribotoxic region comprising a polypeptide and capable of inactivating a ribosome, and ii) a binding region comprising a polypeptide and capable of binding at least one target biomolecule, and, wherein the method comprises the steps of: a) providing a plurality of chimeric proteins, each protein comprising: i) a modified ribotoxic region that comprises at least one amino acid substitution, deletion, insertion, or addition as compared to a corresponding unmodified ribotoxin region, so as to reduce or eliminate ribotoxicity of the modified ribotoxic region, and ii) a binding region comprising a polypeptide and capable of binding at least one target biomolecule; b) screening the plurality of chimeric proteins to identify a chimeric protein with at least one assay-screenable characteristic selected from: target biomolecule binding affinity, target biomolecule binding selectivity, target cell binding affinity, target cell binding selectivity, target cell internalization, and expression level; and c) identifying the amino acid sequences of the polypeptide regions of the chimeric protein identified in step b), or identifying the amino acid sequence of the binding region of the chimeric protein identified in step b), in order to construct one or more chimeric cytotoxic protein comprising an amino acid sequence of the binding region in the identified protein associated with a more ribotoxic form of a modified ribotoxic region present in the identified chimeric protein. i) a ribotoxic region comprising a polypeptide and capable of inactivating a ribosome, and ii) a binding region comprising a polypeptide and capable of binding at least one target biomolecule, and, a) providing a plurality of chimeric proteins, each protein comprising: i) a modified ribotoxic region that comprises at least one amino acid substitution, deletion, insertion, or addition as compared to a corresponding unmodified ribotoxin region, so as to reduce or eliminate ribotoxicity of the modified ribotoxic region, and ii) a binding region comprising a polypeptide and capable of binding at least one target biomolecule; i) a modified ribotoxic region that comprises at least one amino acid substitution, deletion, insertion, or addition as compared to a corresponding unmodified ribotoxin region, so as to reduce or eliminate ribotoxicity of the modified ribotoxic region, and ii) a binding region comprising a polypeptide and capable of binding at least one target biomolecule; b) screening the plurality of chimeric proteins to identify a chimeric protein with at least one assay-screenable characteristic selected from: target biomolecule binding affinity, target biomolecule binding selectivity, target cell binding affinity, target cell binding selectivity, target cell internalization, and expression level; and c) identifying the amino acid sequences of the polypeptide regions of the chimeric protein identified in step b), or identifying the amino acid sequence of the binding region of the chimeric protein identified in step b), in order to construct one or more chimeric cytotoxic protein comprising an amino acid sequence of the binding region in the identified protein associated with a more ribotoxic form of a modified ribotoxic region present in the identified chimeric protein.