The transgene is a 3.9-kb fragment which contains the rat platelet factor 4 (Pf4) promoter followed by exons 2 and 3 of human myc fused to the human estrogen receptor (MER), followed by a 442-base pair region of the HGH poly(A) tail starting from the stop codon of the HGH gene. Two copies of the transgene integrated in this line. Expression of the transgene was detected in bone marrow megakaryocytes.