Structures of normal EpoR and EpoR truncation mutant. The deletion found in the familial erythrocytosis mutation studied here is shown (EpoR FE). There is a frameshift at the coding sequence of amino acid 433 resulting in 17 novel amino acids (black box) followed by a premature stop codon. The 70 terminal amino acids of the normal EpoR (EpoR WT) are deleted in this mutation. The deletion includes 6 of the 9 cytoplasmic tyrosine residues (short vertical lines). The region required for mitogenesis (which includes the box 1 and box 2 domains) is shown in the open box.