The presence of genes represented among a collection of EST sequences is usually represented at a frequency proportional to their expression level from their tissue or cell type of origin. Some genes may be absent from a collection from one tissue, whereas highly expressed genes can be present at 10% or more of the ESTs for a given tissue. This will be reflected in the number of reads that assemble into each contig. Contigs for highly expressed genes will have many reads in them, whereas contigs for genes expressed at low level will have fewer reads.