Thick-section fluorescence in situ hybridization on formalin-fixed, paraffin-embedded archival tissue provides a histogenetic profile

Fluorescence in situ hybridization has become a major tool for analysis of gene and chromosome copy number in normal and malignant tissue. The technique has been applied widely to fresh tissue and dispersed formalin-fixed, paraffin-embedded archival tissue, but its use on sections of archival tissue has largely been limited to sections <6 μ thick. This does not provide intact, uncut nuclei for accurate analysis of gene or chromosome copy number. We report here a method of hybridization to sections >20 μ thick that overcomes these difficulties. Key developments were the use of DNA probes directly labeled with fluorochromes and optical sectioning using laser- scanning confocal microscopy.