The discovery of a brain peptide called orexin reveals how overlooked molecules in our DNA can both explain major diseases and lead to transformative new treatments. Orexin keeps us awake. It starts as a protein so short that standard gene searches overlooked it. Two teams, using two different methods, found it. Their work explained narcolepsy, a disabling sleep disorder. It also led to a new class of drugs now under FDA review .

The peptide’s story raises a hard question for genome science. How many other small, working molecules are still hidden in our DNA because the tools used to search for genes were set up to look for bigger ones?

Orexin is a clear test case for that question. Standard genome annotation pipelines often use length thresholds , commonly 130 amino acids, under the assumption that anything smaller is unlikely to encode functional proteins. As a result, short peptides and functional RNAs can be missed.

A Small Precursor Nearly Missed

Genes carry instructions for making proteins. Messenger RNA carries those instructions out to the cell’s protein factories. To find genes in a long DNA string, computer programs scan for stretches that look like protein-making instructions. Those programs need a size cutoff, or they would flag millions of random short stretches. Many annotation systems ignore short stretches.

The orexin messenger RNA encodes an amino-acid precursor called prepro-orexin. Cells cut this precursor into two smaller active peptides. Orexin A and orexin B are 33 and 28 amino acids long, respectively. At this size, orexin A and B sit close to the line where they can slip through a standard search.

Orexin’s main job is to hold the brain in a steady waking state. The cells that make it sit in a small region of the hypothalamus and send fibers to the brain’s arousal centers. Without that signal, the boundary between waking and sleep breaks down.

Narcolepsy As An Autoimmune Disease

Narcolepsy type 1 causes people to fall asleep suddenly and without warning during the day. Many also experience cataplexy—a sudden, brief loss of muscle control brought on by laughter or surprise, even though they remain fully conscious.

In narcolepsy type 1, the brain loses most or all of its orexin-producing neurons in the hypothalamus. Once the orexin neurons are destroyed, they don’t regenerate, leading to chronic symptoms. This selective loss suggests an immune system cause. The disease is strongly associated with a variant of an immune gene that helps the body distinguish what to attack. Some immune cells specifically target orexin neurons in the blood and spinal fluid, likely after an environmental trigger.

Once the orexin cells are lost, they don’t recover. Orexin itself can’t be given as a pill. Therefore, drugs have been developed that activate the orexin receptor in the brain, restoring the wakefulness signal lost in narcolepsy. For decades, therapy focused on managing symptoms with stimulants, antidepressants or sodium oxybate, but none replaced the lost orexin signal. Recent clinical trials show new drugs can help people with narcolepsy stay awake longer and reduce cataplexy attacks. Their long-term safety and usefulness for other sleep disorders remain to be seen.

What Else The Genome May Hide

Orexin is unlikely to be the only small molecule hiding below the cutoff. Short protein-coding stretches can encode working microproteins of 100 amino acids or fewer. Some even sit inside RNAs long thought to make no protein at all. These include long noncoding RNAs, which may produce their own small peptides. A 2023 study notes that such short stretches have traditionally been left out.

The orexin story highlights how important functional peptides can be overlooked without the right experimental and computational approaches. As genome annotation methods evolve and direct detection of small peptides and RNAs in cells becomes more feasible, many more hidden functional molecules may be discovered. Orexin, nearly missed by standard gene-hunting strategies, has already transformed narcolepsy treatment. It stands as a prime example of what the hidden genome may hold—and why expanding our discovery toolkit could reshape our understanding of biology and disease.