BLAST Search main parameters


Sequence Format
Fasta: A common sequence format offered by most sequence and alignment editors.
Format: A 'greater than' symbol followed by the sequence name. The next next line is the noninterleaved sequence.
Example:
>RL228e1
AGAAGAAGAGGTAGTAATTAGATCTGACAATTTCACGGACAATGCTAAAACTATAATAGTACAGCTGAAAGAACCTGT
AGAAATTAATTGTACAAGACCCCACAACAATACAAGAAGAAGGATAAGTATAGGACCAGGGAGAGCATTTTATGCAAC

Filter (Low-complexity)
Mask off segments of the query sequence that have low compositional complexity, as determined by the SEG program of Wootton & Federhen (Computers and Chemistry, 1993) or, for BLASTN, by the DUST program of Tatusov and Lipman (in preparation). Filtering can eliminate statistically significant but biologically uninteresting reports from the blast output (e.g., hits against common acidic-, basic- or proline-rich regions), leaving the more biologically interesting regions of the query sequence available for specific matching against database sequences.
Filtering is only applied to the query sequence (or its translation products), not to database sequences. Default filtering is DUST for BLASTN, SEG for other programs.
It is not unusual for nothing at all to be masked by SEG, when applied to sequences in SWISS-PROT, so filtering should not be expected to always yield an effect. Furthermore, in some cases, sequences are masked in their entirety, indicating that the statistical significance of any matches reported against the unfiltered query sequence should be suspect.

Filter (Mask for lookup table only)
This option masks only for purposes of constructing the lookup table used by BLAST. The BLAST extensions are performed without masking. This option is still experimental and may change in the near future.

Expect
The statistical significance threshold for reporting matches against database sequences; the default value is 10, such that 10 matches are expected to be found merely by chance, according to the stochastic model of Karlin and Altschul (1990). If the statistical significance ascribed to a match is greater than the EXPECT threshold, the match will not be reported. Lower EXPECT thresholds are more stringent, leading to fewer chance matches being reported. Fractional values are acceptable.

Alignment View
Standard BLAST alignment in pairs of query sequence and database match. For nucleotide, the matches are marked by a pipe symbol ("|") in between query and database sequence. For protein, the identical matches are marked by letter code with "homologous" substitutions (determined by the scoring matrix used) marked by "+" symbol in a line between the query and the database sequence.

Pairwise
The databases alignments are anchored (shown in relation to) to the query sequence in pairwised fashion.

Query-anchored with identities
The databases alignments are anchored (shown in relation to) to the query sequence. Identities are displayed as dots (.), with mismatches displayed as single letter abbreviations.

Query-anchored without identities
Identities are shown as single letter nucleotide abbreviations.

Flat Query-anchored with identities
The 'flat' display shows inserts as deletions on the query. Identities are displayed as dots (.), with mismatches displayed as single letter abbreviations.

Flat Query-anchored without identities
The 'flat' display shows inserts as deletions on the query. Identities are shown as single letter abbreviations.

Hit Table
Simple output with different aligment information separated according to tab delimited fields with field headers are displayed at the top.

Descriptions
Restricts the number of short descriptions of matching sequences reported to the number specified; default limit is 100 descriptions. See also Expect.

Alignments
Restricts database sequences to the number specified for which high-scoring segment pairs (HSPs) are reported; the default limit is 50. If more database sequences than this happen to satisfy the statistical significance threshold for reporting (see Expect), only the matches ascribed the greatest statistical significance are reported.