Medical Genetics The segments retained in the mRNA they called exons and the intervening sequences(A, B and C), which are excised during mRNA processing and maturation, are called introns hybridization poly A-tail messenger RNA DNA mRNA VaMedical Genetics The segments retained in the mRNA they called exons and the intervening sequences (A, B and C), which are excised during mRNA processing and maturation, are called introns