Even in one database, the data are often conflicting each other. For example, the number of human Uni-Gene clusters is about 120,000 [15] . UniGene means unique gene, and is supposed to cluster the transcribed ESTs and mRNA into unique genes. So the number of UniGene clusters should be equal to the number of genes. However, there are only about 25,000 genes in human genome, while the number of UniGene clusters that contain only one sequence is more than 40,000. There must be errors inside. Zhang et al analyzed the results of UniGene clusters [16] and pointed out that most narrowly expressed transcripts (NETs), whose expression is confined to a few tissues, resemble intergenic sequences, and most NETs are singleton clusters containing only one EST or mRNA sequence. So those singleton clusters seem unreliable. The sequences in these clusters may come from non-coding RNA, contamination of pre-mRNA, genomic DNA, non-canonical introns or foreign sources, or simple sequencing errors.