See this page: http://osdir.com/ml/galaxy-source-co.../msg00032.html
So in your case, 255 is for what TopHat considers unique mappings, 3 for mappings with one additional alignment, 1 for 4-9 alignments, 0 for 10+ alignments.
So in your case, 255 is for what TopHat considers unique mappings, 3 for mappings with one additional alignment, 1 for 4-9 alignments, 0 for 10+ alignments.
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