Search is performed as the union of all word present in your query
To make sure a word is in the results, prefix it with +: T4 +NC_000866.4
To make sure a word is absent from results, prefix it with -: T4 -NC_000866.4
To search for a sentence, surrounds it with double quote: "E. coli"
To search only in identifiers, use prefix: ID:255 Many more in the documentation...
Single letter search as disabled by default, if still want to search for it, surround the letter by double-quote : "XXX"