Creating Cascading DropDownLists in ASP.Net without PostBack

hi 

i have 2 dropdown list in my page these are my code

  <asp:DropDownList ID="DDLzone1" runat="server" CssClass="DDlzone" OnSelectedIndexChanged="DDLzone1_SelectedIndexChanged" AutoPostBack="true">
                                    </asp:DropDownList>

 protected void DDLzone1_SelectedIndexChanged(object sender, EventArgs e)
    {





        DDLstore1.Items.Clear();
        DDLstore1.Items.Add(new ListItem("select shopping center", ""));


        DDLstore1.AppendDataBoundItems = true;


        SqlCommand _cmd = new SqlCommand("selectcenter", _cn);
        _cmd.CommandType = CommandType.StoredProcedure;
        _cmd.Parameters.AddWithValue("@RegionID", DDLzone1.SelectedItem.Value);
        try
        {
            _cn.Open();
            DDLstore1.DataSource = _cmd.ExecuteReader();
            DDLstore1.DataTextField = "Centername";
            DDLstore1.DataValueField = "ID";
            DDLstore1.DataBind();


        }
        catch (Exception ex)
        {
            throw ex;
        }
        finally
        {
            _cn.Close();
            _cn.Dispose();
        }
    }

 <asp:DropDownList ID="DDLstore1" runat="server" CssClass="DDLstore" AutoPostBack="true">

when users click on DDLzone1  value of DDlStorre1 will change 

here for both ddl i set autopostback ="true"  for this it change ddlstore when page is post back 

i want when users click on DDLzone1 value of DDLStore1 be change without loading page again