To everyone who contributed to this question, Thank you for all of your useful comments. They certainly clarified the situation for me. Natalie --------------------------------------------------------------- Natalie James, PhD CSIRO Division of Biomolecular Engineering Riverside Corporate Park PO Box 184 North Ryde NSW 2113, AUSTRALIA [log in to unmask] (Ph) 61-2-9886-4885 (Fax) 61-2-9886-4805