"Cell systems can be kept indefinitely at low temperatures [-196° C]," he explains. "We can preserve skin, corneas (for transplantation), and tissues that produce hormones. Strains of cells of parasitic, drug-resistant diseases can be kept in their pre-mutation state so scientists can build a profile of their genetic changes."
What can't be frozen, according to Ashwood-Smith, is an entire living system although frogs can be partially frozen for several weeks."
http://communications.uvic.ca/Ring/9.../freezing.html