A schematic of the human TPO gene from intron 4 to exon 6 and mRNA isoforms. Corresponding portions exhibit the same patterns. TPO-1, TPO-2, and TPO-3 were reported previously. TPO-4, TPO-5, and TPO-6 are newly identified forms. TPO-3, TPO-4, and TPO-5 possess a stop codon that is different from that of TPO-1 and TPO-2 due to the frameshift. TPO-6 contains another stop codon in the inserted sequence that is part of intron 5, which is not spliced out. *End of the erythropoietin-like domain. Horizontal arrows show the regions amplified by RT-nested PCR. The sizes of the PCR products and the sets of primers used are written beside the arrows.