Abstract: In a first aspect of the invention, there is provided a nucleic acid tag comprising: a single-stranded nucleic acid sequence portion having a 5′ end portion and a 3′ end portion; at least two amplification primer binding sequences in said 5′ end and 3′ end portions; internal to these primer binding sequences, at least one marker of about 18 to about 25 nucleotides; and between these markers, a spacer, wherein the spacer has a length sufficient to allow molecular beacons to properly attach to amplification copies of the marker sequences bordering the amplification copy of the spacer and wherein the nucleic acid sequences of primer binding sequences, the marker and the spacer are chosen so as to minimize or prevent secondary structure formation. The said 5′ end and 3′ end portions are preferably protected from degradation. This molecular tag is simple and inexpensive to produce and easy to detect.