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.
- 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
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
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.
- Query Genetic Code
Genetic code to be used in blastx translation of the query.
- 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.
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
- 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
- Hit Table
Simple output with different aligment information separated according to tab delimited fields with field headers
are displayed at the top.
- Restricts the number of short descriptions of matching
sequences reported to the number specified; default
limit is 100 descriptions. See also Expect.
- 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.