Many thanks to Michael for posting the solution to my query wrt which algorithm the PRIMER program uses to calculate Tm's (Breslauer et al [1986] PNAS 83, 3746-3750) and also elaborating on other techniques. Thanks again Michael Alan "It makes supporting BIONET worthwhile" Bleasby SEQNET/EMBnet/BIONET manager DRAL Daresbury Laboratory Warrington WA4 4AD UK