An intervening sequence of the mouse β-globin major gene shares extensive homology only with β-globin genes

THE mouse β-globin major (βmaj) gene is interrupted by two intervening sequences of DNA that divide it into three discontinuous segments1–3. The entire gene, including the coding, intervening and untranslated regions, is transcribed into a colinear 15S mRNA precursor4containing a 5′-cap structure and 3′-poly(A) (refs 5–7). Because mature globin mRNA is significantly smaller (10S) and does not contain these intervening sequences, the 15S precursor must be processed. Such processing presumably accounts at least in part for the reduction in sequence length observed between so-called heterogeneous nuclear RNA (Hn RNA) and cytoplasmic RNA8–10. Intervening sequences seem to occur in a variety of genes and organisms1,11–18, but their function and representation in the genome have been unknown.