在用blastn搜索時(shí),通過輸入的核苷酸序列,我們會(huì)從數(shù)據(jù)庫中得到最為接近的核苷酸序列。盡管我們已經(jīng)在界面上選了相應(yīng)的E值,仍然得到了E值不同的核苷酸序列。這是因?yàn)?/p>
E值代表被比對的兩個(gè)序列不相關(guān)的可能性,E值最低的最有意義,也就是說序列的相似性最大。設(shè)定的E值是我們限定的上限,E值太高的就不顯示了。
以下是英文解釋,有興趣的網(wǎng)友可以看看:
E-value means "expected value", which means:The EXPECTED probability of two random generated sequences appears exactly the way like the two sequences you are BLASTing. That is a statistical concept, if the E-value is high, then this sequence may appear in your BLAST result by random, the probability is exactly the E-value. If the E-value is low, then this result is of high fidelity.
When you input an E-value in BLAST, you set up an upper-bound of E-value, any result below this value will appear. That is the reason why many sequence will apear with different E-values, and the ones with lowest values will appear at the front of all results.