Phytochromes also have the ability to sense light, and cause the plant to grow towards the light this is called phototropism. Janoudi and his fellow coworkers wanted to see what phytochrome was responsible for causing phototropism to occur. So they performed a series of experiments to figure this out that had to start at the beginning. They found that blue light causes the plant Arabidopsis thaliana to exhibit a phototropic response, this curvature is heightened with the addition of red light. They found that five phytochromes are present in the plant, they also found a variety of mutants in which the phytochromes do not function properly. Two of these mutant were very important for this study they are phyA-101 and phyB-1. These are the mutants of phytochrome A and B respectively. The normally functional phytochrome A causes a sensitivity to far red light, and it causes a regulation in the expression of curvature toward the light. Whereas phytochrome B is more sensitive to the red light.