Hair is composed primarily of a protein called keratin.

Unlike living cells in the body, which are constantly being supplied with new material and which contain abundant DNA in their nuclei, the hair shaft itself is largely dead material — the cells of the hair shaft have been keratinized, a process that destroys the cellular structures that make DNA extraction straightforward.

The DNA in hair is present primarily in the hair root and in trace quantities within the shaft, and it degrades significantly over time through chemical processes that break the DNA strands into shorter and shorter fragments.

Hair that has been preserved for more than a century — kept in a museum collection, subjected to varying temperatures and humidity levels, potentially treated with preservatives or cleaning agents — has typically undergone extensive DNA degradation.