What is an E-value in BLAST and how is it interpreted?

The E-value (Expect value) in BLAST represents the number of alignments with similar or better scores expected to occur by chance in a database of a given size. A lower E-value indicates a more significant match (less likely to be random). General interpretation: E-value < 10^-50 indicates near-identical sequences, < 10^-10 suggests homology, < 0.01 may be biologically meaningful but requires further analysis, while values > 1 are likely random matches. E-value depends on database size, sequence length, and alignment score. It is more reliable than percent identity alone for assessing significance.