Cancer researcher Steven Offer, PhD investigates the genetic factors that increase the risk of a toxic reaction to the cancer drug Fluorouracil (5-FU). 5-FU, used since the 1970s to treat many cancers including colorectal, stomach, breast and cervical cancer, can be toxic to certain patients. While many cancer drugs can be toxic, what sets 5-FU apart is that severe illness and death is largely preventable.