Extracellular RNA from leftover pathological specimens that were formalin-fixed are not always of human origin

These tissues when properly stored may be critical resources for researchers. Formalin, which contains ~40% saturated solution of formaldehyde is widely used to fix histochemistry and histopathology preparations, as well as serve as preservatives in embalming solutions used in medical laboratories.

The immersion of tissues in a solution of formalin is expected to inactivate biological activities, protect tissues from microbial growth, and thus preserve the tissue.