Cancer research. Image 2 of 2. Cancer cells. Light micrograph of a precancerous (neoplastic) cell culture for comparison with normal cells (seen in M133/143). This cell culture was exposed to a cancer-causing chemical to test the chemical and study its effects on cell cultures. The cells here are stained much darker, and the growth is denser and more irregular in structure. This is typical of cancerous growth. Photographed in the Health and Pollutants Laboratory of the INRS (the French National Institute of Research and Safety), Nancy, France. Tests are carried out here to establish the toxicity and cancer-causing properties of different materials, chemicals and pollutants.