A biomarker, or biological marker, is a term used to describe any measurable biological indicator or characteristic that can reflect a physiological state, such as a protein, cell, gene, hormone, or other type of molecule. The study and use of biomarkers in medicine is nothing new. Some biomarkers are straightforward and can give a definitive yes or no answer in a wide time range. Other biomarkers are difficult to detect given the minute amount of the markers present, their fleeting presence, or their occurrence in a location that is challenging to access. This means that even when a clinically relevant biomarker has been discovered, it does not necessarily translate to a viable diagnostic test or therapy.