dropdowncheckboxes required field validator in asp.net

<cc2:DropDownCheckBoxes ID="cblCustomerList" runat="server" Width="180px">
    <Style SelectBoxWidth="195" DropDownBoxBoxWidth="160" DropDownBoxBoxHeight="195"/>
                   <Items>
                              <asp:ListItem Text = "Kidneys Donor" Value = "Kidneys Donor"></asp:ListItem>
                        <asp:ListItem Text = "Lungs Donor" Value = "Lungs Donor" >Eye Donor</asp:ListItem>
                        <asp:ListItem Text = "Pancreas Donor" Value = "Pancreas Donor" >Blood Donor</asp:ListItem>
                        <asp:ListItem Text = "One" Value = "1">Heart Donor</asp:ListItem>
                         <asp:ListItem Text = "One" Value = "1" >Liver Donor</asp:ListItem>
                          <asp:ListItem  >Kidneys Donor</asp:ListItem>
                           <asp:ListItem>Lungs Donor</asp:ListItem>
                            <asp:ListItem>Pancreas Donor</asp:ListItem>
                             <asp:ListItem>Intestines Donor</asp:ListItem>
                              <asp:ListItem>Cornea Donor</asp:ListItem>
                               <asp:ListItem>Skin Donor</asp:ListItem>
                                <asp:ListItem>Heart Valves Donor</asp:ListItem>
                                 <asp:ListItem>Tendons Donor</asp:ListItem>
                        </Items>

             </cc2:DropDownCheckBoxes>