The unusual electron microscopic appearance of a hybrid formed between 9S mouse β-globin mRNA and its corresponding cloned gene segment is caused by at least one, and possibly two, intervening sequences of DNA that interrupt the mouse β-globin gene. Such an interpretation is consistent with a paradoxical restriction site pattern previously noted in the gene and with the nucleotide sequence of that portion of the gene that spans both structural and intervening sequences. The large intervening sequence, approximately 550 base pairs in length, occurs in the structural globin sequence and immediately follows the β-globin codon corresponding to amino acid 104. A smaller, putative intervening sequence is located close to the 5' end of the β-globin-coding sequence but may reside beyond its initiation codon. The β-globin gene thus appears to be encoded in two, and possibly three, discontinuous segments.
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