The fruit fly’s internal clock mechanism, like the human mechanism, involves the complex interaction of many genes that produce the organism’s molecular clockworks. Dr. Michael Rosbash explains how fluctuating levels of specific cellular proteins operate in a negative-feedback loop to produce a molecular timekeeping mechanism. This “negative-feedback model” has proved applicable to clocks that are present in nearly every organism studied to date, from bacteria to mice and humans. From the Series:HHMI’s Understanding Biomedical Research Series