A new technique for identifying the precise biological age of human tissue reveals that not all tissues grow old at the same rate. Not all parts of the body age alike, according to Steve Horvath, a geneticist at UCLA’s medical school. Horvath developed a way to determine the biological age of different tissues in the body by looking at DNA methylation, a chemical alteration of genes that has been suggested by previous studies to be a potential biomarker for a cell’s age.

Horvath looked at 8,000 healthy samples of 51 different types of cells and tissues and 6,000 cancerous samples to examine how the aging process affects DNA methylation levels. For the most part, his method accurately tied the biological age (the age predicted from the person’s DNA) to the chronological age of the donor.

Except that some tissues seemed to age far faster than others. “Healthy breast tissue is about two to three years older than the rest of a woman’s body,” according to Horvath. Cancerous cells aged even more rapidly than that. Tumors appeared accelerate the tissue aging process by 36 years, and healthy breast tissue near breast tumors were an average of 12 years older than tissue elsewhere in the body. In contrast, transforming adult human cells into pluripotent stem cells, which reprograms them to act like embryonic stem cells, effectively “resets the cells’ clock to zero,” Horvath says.

The study is online in Genome Biology.