The invention is based on the discovery that certain biarsenical molecules react with specified target sequences, thereby providing a facile means for labeling polypeptides containing the target sequence. The invention is useful in creating stable mammalian cell lines expressing a certain tetracysteine tagged polypeptides, thereby overcoming toxicity associated with native tetracysteine. In addition, the invention allows for orthogonal labeling of polypeptides, thereby allowing for the observation of protein-protein interactions and conformational changes in proteins, for example.