**What is e value in BLAST?**
The e value, also known as the expectation value, is a statistical parameter that measures the likelihood of obtaining a given alignment score by chance in BLAST (Basic Local Alignment Search Tool). It provides an estimate of the number of similar sequences expected to be found by chance in a given database when searching with a particular query sequence.
The e value is an essential metric in BLAST that helps researchers assess the biological significance of a sequence similarity. It helps distinguish matches that are merely coincidental from those that are more likely to have a functional or evolutionary relationship. A lower e value indicates a more significant alignment and suggests a higher probability of a biologically relevant match.
FAQs about e value in BLAST:
1. How is the e value calculated in BLAST?
The e value is calculated based on the size of the database searched, the alignment score, and the length of the query sequence. It is derived using statistical models that consider the probability of obtaining a given score by chance.
2. What is the significance of an e value?
The e value indicates the expected number of sequences with similar alignment scores that would occur by chance. A lower e value suggests a higher likelihood of a meaningful relationship between the query sequence and the hit.
3. What e value cutoff should be used in BLAST searches?
The choice of an appropriate e value threshold depends on the specific study, database size, and the desired level of stringency. A common threshold is 0.01, which implies that an alignment score as extreme as or more extreme than the observed score would occur by chance in only 1% of the database sequences.
4. Can the e value be used to determine the strength of a biological relationship?
The e value alone cannot determine the strength of a biological relationship. It is merely a statistical measure that provides a preliminary assessment of the significance of a sequence similarity. Additional analyses are usually required to confirm and interpret the relationship.
5. How does the e value relate to sequence identity?
The e value and sequence identity are distinct parameters. While sequence identity reflects the similarity between two sequences in terms of identical residues, the e value estimates the likelihood of obtaining a given alignment solely by chance.
6. Can the e value be zero in BLAST?
Technically, the e value cannot be zero in BLAST. However, it can be very close to zero, which suggests an extremely significant alignment between the query and hit sequences.
7. Does a higher e value imply a weaker similarity?
Yes, a higher e value indicates a weaker similarity. A larger e value suggests that the observed alignment score is more likely to have occurred by chance, reducing the confidence in the biological significance of the match.
8. Is the e value affected by the size of the database?
Yes, the size of the database directly affects the e value calculation. A larger database increases the space for chance matches, thus potentially increasing the e value.
9. What is the relationship between significance threshold and e value?
The significance threshold is often set based on an acceptable e value or a desired level of significance. A stricter threshold requires a lower e value, increasing the stringency of the search.
10. Can the e value be used to compare sequences between different searches?
The e value is specific to a particular search and database combination. It cannot be directly used to compare sequences from different searches with distinct databases.
11. Is BLAST the only biological tool that uses e values?
No, e values are commonly used in other sequence analysis tools as well. Tools like HMMER and PSI-BLAST also utilize e values to assess the significance of sequence alignments.
12. Can a high-scoring alignment have a high e value?
Yes, a high-scoring alignment can have a high e value. It could indicate that the observed alignment score is likely to occur by chance within the database, rather than suggesting a meaningful biological relationship.