def split_feature_matrices(x_train, x_test, y_train, y_test, idx):
    """Does the opposite of merge_feature_matrices i.e. when given the 
    train and test matrices for features and labels, splits them into 
    train/active, test/active, train/inactive, test/inactive.

    Parameters:
    	- x_train, x_test, y_train, y_test (2D np.arrays): Feature 
        matrices and label matrices in the sklearn 'X' and 'Y' style.
        - idx (int): a column of the label matrix corresponding to the 
        protein target you wish to test. """
    x_actives_train = x_train[y_train[:,idx]==1]
    x_actives_test = x_test[y_test[:,idx]==1]
    
    x_inactives_train = x_train[y_train[:,idx]!=1]
    x_inactives_test = x_test[y_test[:,idx]!=1]
    
    return x_actives_train, x_actives_test, x_inactives_train, x_inactives_test