When I download CNV SNP array data for ovarian cancer from TCGA data portal, I see some very small numbers like -5, -6 in the "Segment_Mean" column of the segmentation data files. I am very new to genetics, and I don't understand how that much deletion can occur in the gene. We have 2 copies of each gene in our normal cells, and my intuition tells that the minimum loss for a gene in a cell can be -2 (both copies of that gene are lost). So, could someone please explain shortly how this -5, -6 copy number losses are computed?