BLAST (Basic Local Alignment Search Tool) is a widely used bioinformatics program for comparing DNA or protein sequences against a database. One crucial metric provided by BLAST is the E value, which plays a vital role in determining the significance of sequence similarity results.
The meaning of the E value in BLAST
**The E value, also known as the Expect value, in BLAST is a statistical parameter that estimates the number of occurrences of matches with similar scores by chance alone, in a database of the given size.**
In simpler terms, the E value represents the probability of obtaining a hit (sequence similarity match) with a similar or better score purely by random chance, rather than due to actual biological similarity. A lower E value signifies a more significant match, indicating a higher probability of true biological relevance.
The E value is calculated based on the size of the database being searched and the score of the alignment. A smaller E value suggests a more stringent significance threshold, while a larger E value indicates a greater tolerance for random matches.
To better understand the significance of an E value, it helps to consider an example. If a matching sequence has an E value of 0.01, it means that we would expect to find a similar match by chance alone once in every 100 searches. In contrast, an E value of 0.0001 means we would only expect such a match once in every 10,000 searches.
Frequently Asked Questions (FAQs)
Q1: How do I interpret the E value in BLAST?
A1: A lower E value suggests a stronger indication of a true biological match.
Q2: Can the E value be zero?
A2: No, the E value cannot be exactly zero, but it can be very close to zero, indicating an extremely significant match.
Q3: What is considered a good E value?
A3: A good E value depends on various factors, including the size of the database being searched and the specific research question. Generally, an E value below 0.01 is often considered noteworthy.
Q4: Can the E value be greater than 1?
A4: Yes, the E value can be greater than 1, especially when searching small databases. However, a higher E value implies a higher chance of obtaining similar scores by chance and indicates a less significant match.
Q5: Is the E value the only factor to consider in sequence similarity searches?
A5: No, the E value is important but not the sole factor. Factors like alignment length, coverage, and sequence identity also contribute to determining the significance of a match.
Q6: What happens if my E value is high?
A6: A high E value implies a higher likelihood of the match occurring by chance. Consider refining your search parameters or exploring additional analyses to confirm the significance of the match.
Q7: Why is it important to consider the E value in sequence comparisons?
A7: The E value helps researchers assess the statistical significance of sequence similarity results and differentiate between matches that are likely due to chance and those with potential biological relevance.
Q8: Can I compare E values from different BLAST searches?
A8: It is generally not recommended to directly compare E values from different BLAST searches, as they are highly dependent on the size of the database searched.
Q9: Can I compare E values between different types of sequences (e.g., DNA and protein)?
A9: No, E values are not comparable between different types of sequences, as they are based on different scoring systems and statistical models.
Q10: Is a smaller E value more significant than a larger E value?
A10: Yes, a smaller E value indicates a more significant match, with a lower likelihood of occurring by chance.
Q11: Can I use the E value alone to determine if two sequences are homologous?
A11: No, the E value is just one factor to consider. Additional analyses, such as examining alignment length and sequence identity, are necessary to support the conclusion of homology.
Q12: What if I obtain no significant hits with low E values?
A12: If you obtain no significant hits with low E values, it may indicate that your query sequence is dissimilar to any sequences in the database searched. Consider adjusting search parameters or utilizing alternative algorithms or databases.