For over a century now, antibodies have proven to be extremely useful reagents in biomedical research. They are also being tried as therapeutic agents for a number of intractable diseases. Their uses include identification and cloning of new genes from expression libraries, purification and structure-function analysis of proteins, histochemical localization of proteins, as diagnostic markers and/or probes, as targeting agents to deliver drugs and as magic bullets to bind and kill cells specifically among a long list of other uses to which they have been put.