CheckBoxList auto check all

Hi
I have a CheckBoxList
The list goes like this

<asp:CheckBoxList ID="CheckBoxListStr" runat="server" CssClass="CheckBoxListStr"> <asp:ListItem Text="OneSize" Selected="true"></asp:ListItem> <asp:ListItem Text="S" ></asp:ListItem> <asp:ListItem Text="M" ></asp:ListItem> <asp:ListItem Text="L" ></asp:ListItem> <asp:ListItem Text="XL" ></asp:ListItem> <asp:ListItem Text="XXL" ></asp:ListItem> </asp:CheckBoxList>

What I want is that when a user clicks on any other checkbox then the default selected (OneSize) ...