A novel RNA in which the 5' end is generated by cleavage at the poly(A) site of immunoglobulin heavy-chain secreted mRNA.

We describe processed RNA species generated by cleavage at poly(A) sites in the immunoglobulin mu and gamma 2b heavy-chain transcription units. These "amputated transcripts" began at the first or "secreted" poly(A) site and ended at the second or "membrane" poly(A) site. Although they were polyadenylated and apparently spliced, they were largely restricted to the nucleus. Their existence confirms that the heavy-chain mRNAs are derived from RNA cleavage at alternative poly(A) sites.