Cysteine engineered monospecific and bispecific EGFR and/or c-Met FN 3 domain containing molecules comprising one or more free cysteine amino acids are prepared by mutagenizig a nucleic acid sequence of a parent molecule and replacing one or more amino acid residues by cysteine to encode the cysteine engineered FN3 domain containing monospecific or bispecific molecules； expressing the cysteine engineered FN3 domain containing molecules: and recovering the cysteine engineered FN3 domain containing molecule. Isolated cysteine engineered monospecific or bispecific FN3 domain containing molecules may be covalently attached to a detection label or a drug moiety and used therapeutically.
