Expect value. The E-value is a parameter that describes the

number of hits one can “expect” to see by chance when

searching a database of a particular size. It decreases

exponentially with the score (S) that is assigned to a

match between two sequences. Essentially, the E-value

describes the random background noise that exists for

matches between sequences. For example, an E-value of 1

assigned to a hit can be interpreted as meaning that in a

database of the current size, one might expect to see one

match with a similar score simply by chance. This means

that the lower the E-value, or the closer it is to “0”, the

higher is the “significance” of the match. However, it is

important to note that searches with short sequences can be

virtually identical and have relatively high E-value. This

is because the calculation of the E-value also takes into

account the length of the query sequence. This is because

shorter sequences have a high probability of occurring in

the database purely by chance