Scientists reported a genetic signature for a person’s age – to within a decade or so – in a type of white blood cell. The organ that pumps out T cells, the thymus, is gradually replaced with fat tissue as people age. Every time a T cell matures in the thymus it rearranges its DNA to create a molecular receptor that can recognize pathogens and other foreign molecules, leaving loops of excised DNA behind.  The levels of one particular T-cell loop sequence in 195 Dutch volunteers were quantified, and plotted them against their biological ages, which ranged from a few weeks to 80 years old.

They found that they could estimate a person’s age to within 9 years fairly accurately. “The correlation is pretty impressive. How useful it will be in practice as a forensic tool remains to be seen, although there will certainly be forensic cases where it will help as an investigative tool,”.  Scientists doesn’t expect that simply identifying a suspect’s age, but hopes the technique could be combined with other sources of evidence to rule suspects in or out. A potential limitation of the technique is that it could be a poor judge of age in people with HIV, diabetes and other conditions that perturb T cells.