The String::Simrank module allows rapid searches for similarity
between query strings and a database of strings. This module is
maintained by molecular biologists who use it for searching for
similarities among strings representing contiguous DNA or RNA
sequences.  This program does not construct an alignment, but rather
finds the ratio of n-mers (A.K.A. ngrams) that are shared between a
query and database records. The input file should be fasta formatted
(either aligned or unaligned) multiple sequence file.  The memory
consumption is moderate and grows linearly with the number of
sequences and depends on the n-mer size defined by the user.  Using
7-mers, ~20,000 strings each ~1,500 characters in length requires ~50

The module can be used from the command line through the script
examples/ provided.

By default the output is written to STDOUT and represents the
similarity of each query string to the top hits in the the database.
The format is query_id, tab, best match database_id, colon, percent
similarity, space second best match database_id, colon, percent

Simrank statistic: 

For those more comfortable with set theoretic descriptions of
algorithms we provide such description. Note, however, that we only
describe what is calculated, not how, provided description is
algorithmicly less efficient than the one actually implemented in this
package. In words the simrank statistic can be defined as:

Definition: Simrank score is the number of unique n-mers shared
between a query and a databases sequences divided by the smallest
number of unique n-mers of the two.

Let D be a database sequence, and Q be a query sequence. For
a given n (length of the n-mer), we compute the number of unique
n-mers that occur in each of the sequences. Let these numbers be
nmer(D) and nmer(Q), respectively. Further let nvec(D) and nvec(Q) be
the indicator vectors of length |A|^n for each possible n-mer, where
|A| is the size of the alphabet used and

nvec(D)[i] = 1 if D contains n-mer i (in a predefined total enumeration), and 0 otherwise.

Then the Simrank statistic between two sequences can be computed as
Simrank(D,Q) = ---------------------- <nvec(D), nvec(Q)>,

where <.,.> indicated the inner product. Of course, String:Simrank
computes this statistic in more efficient manner than producing
explicit enumeration of all possible n-mers. Instead, of this we only
focus on the n-mers that are actually observed in either D or Q.


To install this module please use the standard procedure by typing the

   perl Makefile.PL
   make test
   make install


Simrank depends on a few packages that are readily available through
CPAN. These are



Standalone executable script is located in the examples
folder of the distribution package. You can use the test data provided
to learn how to use Simrank. In the following example we will build a
database from test_data/db.fasta file and search for the similarity
with sequences in test_data/query.fasta. To do so type:

perl --data ../test_data/db.fasta --query ../test_data/query.fasta 

You should see the following output:

EscCol36	EscCol36:100.00	EscCol36-2:100.00	EscCol43:99.59	EscCol29:99.24	EscCol33:99.17	EscCol10:99.02	EscCol22:97.52EscCo110:97.03	ShgDysen:96.13	EscCol37:93.48	RmlBacte:80.56	AluAgaro:35.45

Our query file contained only one sequence EscCol36. In the output the
id of this sequence comes first, then we see the list of all sequences
in the database sorted by their simrank statistic with the query
sequence. We can limit the simrank identity of the output by using
--minpct flag. For instance:

perl --data ../test_data/db.fasta --query ../test_data/query.fasta --minpct 100

will only output identical sequences.


If you use Simrank in your research please cite: 

Todd Z. DeSantis, Keith Keller, Gary L. Andersen, Alexander
V. Alekseyenko, Niels Larson Simrank: Rapid and sensitive general
purpose k-mer search tool. (in preparation)


simrank - high performance DNA/RNA similarity match utility

Copyright (C) 2007, 2008 Niels Larsen and Todd DeSantis.

This program is free software; you can redistribute it and/or
modify it under the terms of the GNU General Public License
as published by the Free Software Foundation; either version 2
of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.