在用blastn搜索時，通過輸入的核苷酸序列，我們會從數據庫中得到最為接近的核苷酸序列。盡管我們已經在界面上選了相應的E值，仍然得到了E值不同的核苷酸序列。這是因為

E值代表被比對的兩個序列不相關的可能性，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.