In contrast to homobifunctional reagents, heterobifunctional reagents contain two dissimilar functional groups of different speciŠcities. These two reactive functionalities may be any combination of the conventional group-selective moieties discussed in earlier chapters. For example, one end of the cross-linker may be selective for an amino group while the other end directed to a sulfhydryl group. As categorized in Table 6.1, a variety of combinations is possible; thus, different amino acids in proteins as well as DNAs can be conjugated together. A comprehensive list of these heterobifunctional cross-linkers can be found in Appendix D. It should be stressed again, however, that with few exceptions, no reagent is absolutely group speciŠc. Cross-reactivity exists between the various classes of heterobifunctional cross-linkers presented in the classiŠcation, which is a guide for selectivity rather than speciŠcity. By taking advantage of the differential selectivity of the different functional groups, cross-linking reactions can be controlled selectively and sequentially.