Web client tells the SOAP server that it wants to be informed whenever _new_ Cu-containing proteins are indexed in the MDB. Gives as callback the URL: _http://www.example.com/newcu.jsp_, and wants to recieve data in RSS format, using the HTTP POST method. MDB Web services know how to use POST and format data in RSS, so a registration ID is returned. ID can be used to unregister the callback, or modify it. The following week, new protein structures are indexed. Five new Cu-containing proteins are found. MDB uses the registered URL and POSTs a RSS formatted document with information on the new Cu-containing proteins. Callback is repeated next week unless unregistered, modified, or if an error condition was encountered the previous week (e.g. a critical HTTP error).