CheckboxList Control: How to ensure this ALL checkbox is checked onload

I have this ALL option in my control below. How to ensure onload, the option "ALL" is checked ? Thanks
 
 <asp:CheckBoxList ID="CheckBoxList1" runat="server" DataSourceID="YearSource"
        DataTextField="YR" DataValueField="YR" RepeatColumns="12"
        AutoPostBack="True" AppendDataBoundItems="true">
           <asp:ListItem Value="%">ALL</asp:ListItem>
    </asp:CheckBoxList>
...