Blastn中的E值
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.
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