Expect (E) value

The Expect (E) value refers to the number of hits you can expect are made by chance when searching for matches to a certain DNA base sequence or amino acid sequence in a particular database (query sequence). It decreases exponentially as the Score (S) increases. For instance an E value of 1 will tell you to expect one match of the query sequence was made by chance in the database you have searched. The lower the E-value, or the closer it is to zero, the more significant the match is. A factor that affects E values is length of the query sequence as shorter ones have more chance of making a match in the database by chance than longer ones so shorter query sequences normally have higher E values.