6. Tissue microdissection and DNA extraction. The formalin-fixed, paraffin-embedded tissue blocks of the initial biopsy and biopsy from the follow-up visit will be obtained from the Department of Anatomical Pathology, the Vancouver General Hospital. at the time of clinical change or at 24 months follow-up visit will be requested for processing. Ten 10-µm thick sections for DNA sample, 2 4-µm unstained tissue sections for quantitative histological analysis. Areas of dysplasia on the thick sections will be identified and microdissected. The DNA sample from the epithelium will be extracted and used for the LOH analysis.