By deciphering the molecular signatures of millions of mouse cells, Junyue Cao has found that aging is not haphazard wear and tear but rather a "remodeling of the cell society." Cao, a cell biologist at Rockefeller University in New York City, analyzed gene expression in millions of mouse cells from different life stages. He was surprised to find that "changes in aging are not universal across all the cells." Using technology that offers a systemwide view of the aging process in mice, Cao has outlined discrete stages of aging, akin to those of embryonic development, that are defined by changes in molecular signals and specific cell populations. In humans, the process likely begins before age 30.

Cao's interest in aging began in high school in Hebei, China, when he became acutely aware that his grandparents and parents were not going to live forever. While many teenagers awakening to mortality might turn to poetry or self-destructive behavior, Cao turned to science. Finding a way to slow aging became his lifelong goal, and it's why he chose biology as his major at Peking University in Beijing.

Initially, Cao assumed that the deterioration of particular proteins and protein networks was responsible for aging, in line with the prevailing model. But then after college, when he was working in a lab trying to identify those proteins, he realized that a daunting number of them were associated with aging, and that their effects depended on the type of cell in which they were operating. It was a picture both more complex and more organized than he had thought.

Cao decided he needed data -- lots of it -- on thousands of molecular changes across hundreds of cell types. As a graduate student, he developed a high-throughput technology that could quantify these dynamics in embryonic development. When he started his own lab at Rockefeller in 2020, he put this technology to work on aging.

In one series of experiments, Cao and his team processed 21 million cells, sampled from 14 tissues or organs in about 50 male and female mice at five life stages, and built a dataset of gene expression for each cell. Each stage was marked by a dramatic decline in or expansion of specific cell types. Two of his landmark papers, published in 2025 and 2026 in Science, point to a radical redistribution of the cells that make up the body as mammals age, and describe some of the epigenomic instructions that guide this process. "There are molecular changes and maybe some other changes in aging," Cao sai...