Bind List object to datagrid combobox

Question

How to bind list of integers to Datagridcombobox column.

I tried with the following code, It shows empty dropdown

 

<DataGridComboBoxColumn x:Name="cbUnitLevel" Header="Unit Level" ItemsSource="{Binding Path=UnitLevels,Mode=TwoWay}" SelectedValueBinding="{Binding UnitLevels,Mode=TwoWay}" SelectedItemBinding="{Binding Path=UnitLevels,Mode=TwoWay}" DisplayMemberPath="{Binding UnitLevels}"/>