Specifically, uracil, when irradiation occurs in water ice. This is consistent with the nucleobases found on the Murchison meteorite; among the interesting molecules found on comets we can count at least the (simplest) amino acid glycine returned to Earth from Wild-2. More here.
This strengthens the argument for the origin of life by delivery of replicator molecules to the prebiotic Earth by comets and/or meteors (is this even controversial at this point?) but raises the bar on the kind of chemistry we'll need to see on those bodies to believe that we're seeing evidence of replicators from elsewehre, von Neumann probes or otherwise.
Michel Nuevo, Stefanie N. Milam, and Scott A. Sandford. Astrobiology. April 2012, 12(4): 295-314. doi:10.1089/ast.2011.0726.