The current view of carcinogenesis is that normal cells are
transformed into tumour cells by mutations that activate
oncogenes, silence tumour suppressor genes or trigger genetic
instabilities. As a consequence, every tumour is the result of a
unique evolutionary process that begins in its individual host
and ends with the elimination of the tumour or the death of the
host. Two recent studies, however, suggest that tumour cells can
behave like infectious agents and move from one host to another
1,2 It has long been suspected that canine transmissible
venereal tumour (CTVT) is transferred between dogs by
implantation of tumour cells from donor to the recipient, where
the tumour grows as an allograft 3 . Several lines of evidence
provided indirect support for this hypothesis. The tumour can
only be induced by the implantation of whole tumour cells but
not by cell extracts or dead cells. Normal canine cells have 78
chromosomes but karyotypes of tumour cells isolated from
different animals have shown a characteri