<s>
In	O
machine	O
learning	O
and	O
data	B-Application
mining	I-Application
,	O
a	O
string	B-General_Concept
kernel	I-General_Concept
is	O
a	O
kernel	O
function	O
that	O
operates	O
on	O
strings	O
,	O
i.e.	O
</s>
<s>
String	B-General_Concept
kernels	I-General_Concept
can	O
be	O
intuitively	O
understood	O
as	O
functions	O
measuring	O
the	O
similarity	O
of	O
pairs	O
of	O
strings	O
:	O
the	O
more	O
similar	O
two	O
strings	O
a	O
and	O
b	O
are	O
,	O
the	O
higher	O
the	O
value	O
of	O
a	O
string	B-General_Concept
kernel	I-General_Concept
K(a, b )	O
will	O
be	O
.	O
</s>
<s>
Using	O
string	B-General_Concept
kernels	I-General_Concept
with	O
kernelized	O
learning	O
algorithms	O
such	O
as	O
support	B-Algorithm
vector	I-Algorithm
machines	I-Algorithm
allow	O
such	O
algorithms	O
to	O
work	O
with	O
strings	O
,	O
without	O
having	O
to	O
translate	O
these	O
to	O
fixed-length	O
,	O
real-valued	O
feature	B-Algorithm
vectors	I-Algorithm
.	O
</s>
<s>
String	B-General_Concept
kernels	I-General_Concept
are	O
used	O
in	O
domains	O
where	O
sequence	O
data	O
are	O
to	O
be	O
clustered	B-Algorithm
or	O
classified	B-General_Concept
,	O
e.g.	O
</s>
<s>
in	O
text	B-Algorithm
mining	I-Algorithm
and	O
gene	O
analysis	O
.	O
</s>
<s>
data	O
are	O
elements	O
of	O
a	O
vector	O
space	O
)	O
,	O
using	O
a	O
string	B-General_Concept
kernel	I-General_Concept
allows	O
the	O
extension	O
of	O
these	O
methods	O
to	O
handle	O
sequence	O
data	O
.	O
</s>
<s>
In	O
bioinformatics	O
string	B-General_Concept
kernels	I-General_Concept
are	O
used	O
especially	O
to	O
transform	O
biological	O
sequences	O
such	O
as	O
proteins	O
or	O
DNA	O
into	O
vectors	O
for	O
further	O
use	O
in	O
machine	O
learning	O
models	O
.	O
</s>
<s>
An	O
example	O
of	O
a	O
string	B-General_Concept
kernel	I-General_Concept
used	O
for	O
that	O
purpose	O
is	O
the	O
profile	O
kernel	O
.	O
</s>
<s>
satisfying	O
some	O
conditions	O
(	O
being	O
symmetric	B-Algorithm
in	O
the	O
arguments	O
,	O
continuous	O
and	O
positive	O
semidefinite	O
in	O
a	O
certain	O
sense	O
)	O
.	O
</s>
<s>
For	O
several	O
relevant	O
algorithms	O
,	O
data	O
enters	O
into	O
the	O
algorithm	O
only	O
in	O
expressions	O
involving	O
an	O
inner	O
product	O
of	O
feature	B-Algorithm
vectors	I-Algorithm
,	O
</s>
<s>
hence	O
the	O
name	O
kernel	B-Algorithm
methods	I-Algorithm
.	O
</s>
