A parasitic cancer or transmissible cancer is a cancer cell or cluster of cancer cells that can be transmitted from animal to animal.

They are also known as "clonally transmissible cancers".

Cancer is not normally a contagious disease, but there are three known exceptions in dogs, Tasmanian devils and Syrian hamsters. These cancers have a relatively stable genome as they are transmitted. Because of their transmission, it was initially thought that these diseases were caused by the transfer of oncoviruses, in the manner of cervical cancer caused by HPV.

It has been suggested that animals that have undergone population bottlenecks are at greater risks of contracting transmissible cancers.

Transmissible cancers are rare in humans. A malignant fibrous histiocytoma was contracted from a patient by a surgeon when he injured his hand during an operation.

Paul Greengard (born December 11, 1925) is an American neuroscientist best known for his work on the molecular and cellular function of neurons. In 2000, Greengard, Arvid Carlsson and Eric Kandel were awarded the Nobel Prize for Physiology or Medicine for their discoveries concerning signal transduction in the nervous system. He is currently Vincent Astor Professor at Rockefeller University, and serves on the Scientific Advisory Board of Cure Alzheimer's Fund.

Greengard's research has focused on events inside the neuron caused by neurotransmitters. Specifically, Greengard and his fellow researchers studied the behavior of second messenger cascades that transform the docking of a neurotransmitter with a receptor into permanent changes in the neuron. In a series of experiments, Greengard and his colleagues showed that when dopamine interacts with a receptor on the cell membrane of a neuron, it causes an increase in cyclic AMP inside the cell. This increase of cyclic AMP, in turn activates a protein called protein kinase A, which turns the function other proteins on or off by adding phosphate groups in a reaction known as phosphorylation. The proteins activated by phosphorylation can then perform a number of changes in the cell: transcribing DNA to make new proteins, moving more receptors to the synapse (and thus increasing the neuron's sensitivity), or moving ion channels to the cell surface (and thus increasing the cell's excitability). He was awarded the Nobel Prize in 2000 "for showing how neurotransmitters act on the cell and can activate a central molecule known as DARPP-32".