In situ hybridization techniques, initially developed by J. G. Gall and M. L. Pardue and John et al., have proved to be powerful tools for determining the chromosomal location of hybridized nucleic acids. Early in situ studies used radioactive RNA or DNA probes that were labeled with H or I, and the sites of hybridization were detected autoradiographically. In 1982, a new method was developed to localize DNA sequences hybridized in situ to chromosomes. This procedure utilizes a biotin-labeled analog of tymidine (TTP) that can be incorporated enzymatically into DNA probes by nick translation. The technique using total genomic DNA as a probe is called genomic in situ hybridization (GISH). In this technique, genomic DNA from one species is used as the labeled probe, while unlabeled DNA from the other species under test is used as the competitor at a much higher concentration. Several methods for labeling DNA probes for nonradioactive in situ hybridization have become available over the past few years.