Is it ensure, insure, or assure?
precursor pre·cur·sor (prĭ-kûr'sər, prē'kûr'sər)
One that precedes and indicates something to come.
One that precedes another; a forerunner or predecessor.
A biochemical substance, such as an intermediate compound in a chain of enzymatic reactions, that gives rise to a more stable or definitive product.