The hypothesis makes two specific predictions amenable
to experimental test. DNA complementary to viral
RNA should appear after in fection and therefore should be
detectable by molecular hybridization . Suggestive but
not decisive cxperiments supporting this prediction have
been reported Future ther Temin invokes the existence
of an enzyme that can carry out a revcrsal of transcription
by catalysing the synthesis of DNA on un RNA template.
Evidence for such an enzyme has been presented recently
by Baltimore' and Temin and Mitzutani , who found a
DNA· polymerizing activity in both avian and muine
tumour virllses. Tho enzyme was detected by the incorporation
of tritium-labelled thymidine triphosphate
(3H·TTP) into all. acid -insoluble product t hat can be
destroyed by deoxyribonuclease. Maximal activity
required t he presence of all four deoxyribosid e triphos.
phates and magnesium. The fact that the activity is
inhibited by ribonuclease implies that the RNA of the
virion is necessary for the reaction .
These find ings are clearly pregnant with implications
for the molecular details of viral oncogcnesis. Their
potential importance demands quick confirmation and
extension, a task t he present wolk undertook to fulfil.