Richard J. Roberts and Phillip A. Sharp made their amazing discovery of split genes in the adenovirus when they examined a hybrid between a viral mRNA and its template DNA in the electron microscope. They observed that the mRNA was not encoded as a coline or segment on the DNA molecule. Instead, remarkable loops of unhybridized DNA (A, B and C in figure) were seen. The interpretation was that mature messenger RNA was derived from four discontinuous segments on the viral DNA. The segments retained in the mRNA are called exons and the intervening sequences (A, B and C), excised during mRNA maturation, are called introns.