diff --git a/gklearn/kernels/__init__.py b/gklearn/kernels/__init__.py
index 6ffef06..753715e 100644
--- a/gklearn/kernels/__init__.py
+++ b/gklearn/kernels/__init__.py
@@ -7,7 +7,6 @@ __version__ = "0.1"
 __author__ = "Linlin Jia"
 __date__ = "November 2018"
 
-from gklearn.kernels.metadata import GRAPH_KERNELS, list_of_graph_kernels
 
 from gklearn.kernels.graph_kernel import GraphKernel
 from gklearn.kernels.common_walk import CommonWalk
@@ -24,6 +23,8 @@ from gklearn.kernels.path_up_to_h import PathUpToH
 from gklearn.kernels.treelet import Treelet
 from gklearn.kernels.weisfeiler_lehman import WeisfeilerLehman, WLSubtree
 
+from gklearn.kernels.metadata import GRAPH_KERNELS, list_of_graph_kernels
+
 # old version.
 from gklearn.kernels.commonWalkKernel import commonwalkkernel
 from gklearn.kernels.marginalizedKernel import marginalizedkernel
@@ -32,4 +33,4 @@ from gklearn.kernels.spKernel import spkernel
 from gklearn.kernels.structuralspKernel import structuralspkernel
 from gklearn.kernels.untilHPathKernel import untilhpathkernel
 from gklearn.kernels.treeletKernel import treeletkernel
-from gklearn.kernels.weisfeilerLehmanKernel import weisfeilerlehmankernel
\ No newline at end of file
+from gklearn.kernels.weisfeilerLehmanKernel import weisfeilerlehmankernel
diff --git a/gklearn/kernels/metadata.py b/gklearn/kernels/metadata.py
index 188fc56..e3a5fde 100644
--- a/gklearn/kernels/metadata.py
+++ b/gklearn/kernels/metadata.py
@@ -5,23 +5,35 @@ Created on Fri Nov  6 10:11:08 2020
 
 @author: ljia
 """
+from gklearn.kernels.common_walk import CommonWalk
+from gklearn.kernels.marginalized import Marginalized
+from gklearn.kernels.sylvester_equation import SylvesterEquation
+from gklearn.kernels.conjugate_gradient import ConjugateGradient
+from gklearn.kernels.fixed_point import FixedPoint
+from gklearn.kernels.spectral_decomposition import SpectralDecomposition
+from gklearn.kernels.shortest_path import ShortestPath
+from gklearn.kernels.structural_sp import StructuralSP
+from gklearn.kernels.path_up_to_h import PathUpToH
+from gklearn.kernels.treelet import Treelet
+from gklearn.kernels.weisfeiler_lehman import WLSubtree
+
 
 # The metadata of all graph kernels.
 GRAPH_KERNELS = {
 	### based on walks.
-	'common walk': '',
-	'marginalized': '',
-	'sylvester equation': '',
-	'fixed point': '',
-	'conjugate gradient': '',
-	'spectral decomposition': '',
+	'common walk': CommonWalk,
+	'marginalized': Marginalized,
+	'sylvester equation': SylvesterEquation,
+	'fixed point': FixedPoint,
+	'conjugate gradient': ConjugateGradient,
+	'spectral decomposition': SpectralDecomposition,
 	### based on paths.
-	'shortest path': '',
-	'structural shortest path': '',
-	'path up to length h': '',
+	'shortest path': ShortestPath,
+	'structural shortest path': StructuralSP,
+	'path up to length h': PathUpToH,
 	### based on non-linear patterns.
-	'weisfeiler-lehman subtree': '',
-	'treelet': '',
+	'weisfeiler-lehman subtree': WLSubtree,
+	'treelet': Treelet,
 	}