Making Readonly for HtmlEditorExtender

<asp:TextBox ID="TextBoxPassLimitDescription " TextMode="MultiLine" runat="server" Height="700px" Width="100%"></asp:TextBox>
<ajax:HtmlEditorExtender ID="htmlEditorExtender" TargetControlID="TextBoxMessage" runat="server"> 
 <Toolbar>
  <ajax:Subscript /> 
 <ajax:Superscript /> 
 </Toolbar> 
</ajax:HtmlEditorExtender>

Now I want to set the TextBoxPassLimitDescription textbox to readonly/non-editable mode. It seems 
HtmlEditorExtender does not let you change the textbox to readonly/non-editable state and always 
changes it to editable state.Kindly let me know what to do.

Thanks in advance