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<h1>Purge Manual</h1>
<p>Purge removes redundant sequences from a FASTA file. This is recommended
in order to prevent highly similar sequences distorting the search
for motifs. Purge was written by Andy Neuwald and is described in more
detail in Neuwald <i>et al.</i>, "Gibbs motif sampling: detection of bacterial outer membrane protein repeats", Protein Science, <b>4</b>:1618--1632, 1995.
</p>
<p>Purge works with either DNA or protein sequences and creates
an output file such that no two sequences have a (gapless) local alignment
score greater than a threshold specified by the user. The alignment
score is based on the BLOSUM62 matrix for proteins, and on a +5/-1
scoring scheme for DNA. Purge can also be used to mask tandem repeats.
It uses the XNU program for this purpose.
</p>
<h2> Using Purge </h2>
<ul>
<li>
For typical-length protein sequences, a reasonable purge threshold is
around 150.
To purge a file of sequences named "examples/lipocalin.s", do the following
command. The purged sequences will be put in file "examples/lipocalin.s.b150".
You should experiment with different purge thresholds depending on your
particular needs. Depending on the application values between 60 and 250
may be appropriate.
<pre>
purge examples/lipocalin.s 150
</pre>
</li>
<li>
For DNA sequences of about 100 bp, a reasonable purge threshold
is about 175. As the sequences become longer, higher thresholds will be
required to avoid removing too many sequences.
To purge a file of sequences named "examples/crp0.s", do the following
command. The purged sequences will be put in file "examples/crp0.s.b175".
<pre>
purge examples/crp0.s 175
</pre>
</li>
</ul>
<h2> Purge Usage Message </h2>
<ul>
<li>
If you type "purge" at the command line, you will receive the following
usage message.
</li>
</ul>
<pre>
usage: purge file score <options>
options:
[-n] - sequences are DNA (default: protein)
[-b] - use blast heuristic method (default for protein)
[-e] - use an exhaustive method (default for DNA)
[-q] - keep first sequence in the set
[-x] - use xnu to mask protein tandem repeats
Purge creates an output file from the input file such that
no two sequences have local alignment score greater than <score>.
The output file is named <file>.<score>.
Substitution matrices: BLOSUM62 (protein), +5/-1 (DNA).
</pre>
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