Intron-exon organization of the human TβR-I: identification and localization of an N-terminal deletion in JK cell TβR-I cDNA. JK cell total RNA was subjected to RT-PCR analysis to facilitate the cloning and isolation of full-length JK cell TβR-I cDNAs as described in Materials and Methods. Shown is the transcriptional start site (pos. −236) and the intron-exon organization of the human TβR-I as described recently by Vellucci and Reiss.63 As depicted, the TβR-I is comprised of 9 exons spanning ∼31 kb of genomic DNA sequence. Sequencing of 13 JK cell TβR-I cDNA clones generated by RT-PCR demonstrated the presence in 12 clones of a 178-bp deletion, beginning at position −81 relative to the initiating methionine and concluding at position +96. This deletion eliminates the initiating methionine and the final 178 bp of exon 1 of the TβR-I in JK cells.