G'day, You might want to check out Hinton et al, NUC. Acid Res. 10, 1877 (1982), & Harrison and Zimmerman, Nuc. Acid Res. 12, 8235 (1984). Alan Distribution: INTERNET:phjf at ariel.ucs.unimelb.EDU.AU INTERNET:phjf at ariel.ucs.unimelb.EDU.AU METHODS-AND-REAGENTS >INTERNET:methods at net.bio.net