According to the two-hit model, two events are necessary for the retinal cell or cells to develop into tumors.

Throughout the 1920s' work continued with these mice as model organisms for research into tumors and genetics.

These cells normally disappear, but very rarely they remain and grow into tumors.

In that treatment, viruses containing the desired gene are injected directly into tumors.

And one bad cell can replicate wildly, turning into tumors.

Usually considered benign but can grow into malignant tumors.

The hope is that the special cold cells, injected into tumors, could actually kill the cancer cells.

It has been worked on for years for delivering drugs through the skin or into tumors.

This allows the cancer cells to develop into tumors.

These little balloons could be filled with cancer-fighting drugs and injected into solid tumors.