Schematic illustration showing the major isoform of Mpl (hMpl-P)38 and the v-Mpl fusion product. Mpl has two CRMs, with the distal one containing an insertion of 50 residues. The v-Mpl product is derived from the MPLV retrovirus and is a fusion protein containing rearranged env sequences fused to truncated Mpl (see text). The conserved cysteine residues and WSXWS motif characteristic of the CR family are indicated by horizontal lines.