Fishing for genes. Fluorescence in situ hybridization (FISH) probes are strands of DNA that have been labeled with fluorescent dye molecules. The probes bind uniquely to complementary strands of chromosomal DNA, thus pinpointing the positions of target DNA sequences. In this example, one probe, whose fluorescence signal is shown in red, binds specifically to a gene (DSRAD) that codes for an important RNA-modifying enzyme. A second probe, whose signal appears in green, binds to a marker sequence whose location was already known. The previously unknown location of the DSRAD gene was thus accurately mapped to a narrow region on the long arm of chromosome 1.

To Know Ourselves was prepared at the request of the U.S. Department of Energy, Office of Health and Environmental Research, as an overview of the Human Genome Project.