the theory that cancer cells themselves could be an infective agent (the allograft theory) was first offered in 2006 by pearse, swift and colleagues, who analysed dftd cells from devils in several locations, determining that all dftd cells sampled were genetically identical to each other, and genetically distinct from their hosts and from all other individual tasmanian devils whose genetics had been studied; this allowed them to conclude that the cancer originated from a single individual and spread from it, rather than arising repeatedly, and independently. twenty-one different subtypes have been identified by analysing the mitochondrial and nuclear genomes of 104 tumours from different tasmanian devils. researchers have also witnessed a previously-uninfected devil develop tumours from lesions caused by an infected devil's bites, supporting the contention that the disease is spread by allograft, with transmission via biting, scratching, and aggressive sexual activity between individuals. during biting, infection can spread from the bitten devil to the biter.