在用blastn搜索时，通过输入的核苷酸序列，我们会从数据库中得到最为接近的核苷酸序列。尽管我们已经在界面上选了相应的E值，仍然得到了E值不同的核苷酸序列。这是因为

E值代表被比对的两个序列不相关的可能性，E值最低的最有意义，也就是说序列的相似性最大。设定的E值是我们限定的上限，E值太高的就不显示了。

以下是英文解释，有兴趣的网友可以看看：

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.