One of the most significant shifts in emphasis in recent years on the MCAT was the announcement by AAMC of greater emphasis on genetics and molecular biology on the exam. This is a shift that practically guarantees the presence of such a passage on your exam. More than ever, students need to be comfortable with discussion of the basic repertoire of techniques in molecular genetics. One such technique is in situ hybridization. In this technique a labeled complementary DNA or RNA strand is used to localize a specific DNA or RNA sequence in a portion or section of tissue (in situ). When a labeled DNA or RNA strand is used in this way, it is refered to as a probe. Probes may be labeled with radioisotope, antigen, enzyme, or fluorescent dye, to be detected respectively through associated schema such as autoradiography or immunohistochemical methods, for example. Historically, the techniques of in situ hybridization have been very important in chromosomal localization experiments.