C# - event not firing

<asp:Button ID="btn1"  GroupName="cmdOpt" Name="opt" runat="server" Text="1" TabIndex="1" onclick="btn1_Click"   />

protected void btn1_Click(object sender, EventArgs e)
    {
        Session["strBtn"] = strBtn;
        // Response.Redirect("Default2.aspx",false);
    }
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       <asp:DropDownList ID="cboRegion" runat="server" Width="83" OnTextChanged="cboRegion_OnTextChanged" >
            <asp:ListItem Text="Region 1" Value="1"  />
            <asp:ListItem Text="Region 2" Value="2"  />
            <asp:ListItem Text="Region 3" Value="3"  />
        </asp:DropDownList>

    protected void cboRegion_OnTextChanged(object sender, EventArgs e)
    {
        Session["Year"] = "3333";
        // Response.Redirect("Default2.aspx",false);
    }

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