 However, previous methods usually  used the GO hierarchy to refine the prediction results of multiple classifiers, or flattened the hierarchy into a function-function similarity kernels. Only rarely has the hierarchy been used as a second layer of the network connecting the protein network with functional annotations .
<clarity>  However, previous methods usually  used the GO hierarchy to refine the prediction results of multiple classifiers, or flattened the hierarchy into a function-function similarity kernels. Only rarely has the hierarchy been used as a second layer of the network connecting the protein network with functional annotations .
 However, previous methods usually used the GO hierarchy to refine the prediction results of multiple classifiers, or flattened the hierarchy into a function-function similarity kernel. No study has taken the GO hierarchy into account together with the protein network as a two-layer network model .