Scientists estimate the true biological age of an organism (as opposed to their chronological age) by measuring biomarkers of aging. A biomarker of aging is a biological marker that changes in a predictable way as an organism ages, and can therefore be measured to assess the extent of aging in an organism’s tissues. Measuring biological age is challenging because there are many biomarkers of aging that could be taken into account, and said biomarkers may not change at the same rate from one organ to the next. This means that a single organism could effectively have multiple biological ages throughout their tissues. Some of the main mechanisms used as biomarkers of aging involve modifications to the DNA, such as DNA methylation, histone modification and telomere shortening. Others include levels of background inflammation, mitochondrial function, cellular senescence, levels of various proteins and signalling molecules, and many others.