Many applications of genetic probes for determination of alterations in tumor tissue have recently been described. They add diagnostic capabilities that should be easily and rapidly extended to analysis of cytology samples. Probes for chromosome enumeration and for delineation of presence, absence, or copy number of specific genes or localized chromosomal regions are widely available commercially. When used in combination with other markers for nuclear, cytoplasmic, or cell membrane components, such probes can greatly increase the information that is retrievable from very limited numbers of cells.
The underlying technique that enables specific genetic analysis by probe detection is DNA hybridization. The principles of DNA hybridization are, first, that DNA single strands can bond (or anneal) to form a duplex or double-stranded DNA configuration. Second, when the base sequence between the strands shows greater complementarity, the bonding between the two strands is tighter. Thus, if native DNA is denatured to a singlestranded configuration and incubated with a specific labelled DNA, i.e., the probe (also single-stranded), the result will be complementary binding of strands: the hybridization step. The specific segments of DNA, whether genes or chromosomal regions, can then be tagged with radioactive labels or, preferably for in situ studies, directly with fluorochromes or with labels such as biotin or digoxigenin. The latter indirect, nonisotopic labelled probes are then reacted with avidin or antibody, which in turn are linked to either an immunoenzymatic or fluorescent marker. What is critical to interpretation is that the label will be localized to the chromosomal or intranuclear location where that native DNA sequence normally resides.