Cysteine engineered antibodies comprising a free cysteine amino acid in the heavy chain or light chain are prepared by mutagenizing a nucleic acid sequence of a parent antibody and replacing one or more amino acid residues by cysteine to encode the cysteine engineered antibody； expressing the cysteine engineered antibody； and isolating the cysteine engineered antibody. Certain highly reactive cysteine engineered antibodies were identified by the PHESELECTOR assay. Isolated cysteine engineered antibodies may be covalently attached to a capture label, a detection label, a drug moiety, or a solid support.
