BLAST search results. Part of the results from a BLAST search of the nonredundant (nr) protein sequence database using the sequence of ribose 5-phosphate isomerase (also called phosphopentose isomerase, Chapter 20) from E. coli as a query. Among the thousands of sequences found is the orthologous sequence from humans, and the alignment between these sequences is shown (highlighted in yellow). The number of sequences with this level of similarity expected to be in the database by chance is 2 × 10−27 as shown by the E value (highlighted in red). Because this value is much less than 1, the observed sequence alignment is highly significant.