Variable amounts of proteins or protein fragments may survive in fossils for thousands and occasionally millions of years. The quantities are generally too small to determine accurate amino acid sequences, but even these small amounts may be immunologically detectable and may retain useful genetic information. A very sensitive solid phase radioimmunoassay has been used to analyse fossil proteins, particularly albumin and collagen, the most abundant animal proteins and those most likely to be found in fossil skin, muscle, bones and teeth. Species-specific proteins have been identified in the skin and muscle of mammoth, quagga and Tasmanian wolf, and in the bones of the extinct mastodon and Steller's sea cow. Specific albumin has also been detected in bloodstains on ancient stone weapons and in fossilized urine from the middens of cave-dwelling pack rats, porcupines and hyraxes.