Is it ensure, insure, or assure?
Old English mearcere "writer, notary" (glossing Latin notarius "clerk"), literally "one who marks," agent noun from mark (v). Not found again until late 15c., hence modern use is perhaps a separate formation. Meaning "monument stone" is from 1888. Meaning "felt-tipped marker pen" is from 1951, so called because their purpose was to "highlight" text.
marker mark·er (mär'kər)
One that marks or serves as a mark.
A physiological substance, such as human chorionic gonadotropin or alpha-fetoprotein, that may indicate disease when present in abnormal amounts in the serum, as that caused by a malignancy. Also called biomarker.
A genetic marker.