Suppression of thrombocytosis by adding back Mpl wild-type alleles or the Δ60 knockin allele. (A) Platelet counts (error bars = SD) in peripheral blood from nontransgenic () or transgenic (■) mice that are homozygous (Mpl^−/−), heterozygous (Mpl^−/+), or wild-type (Mpl^+/+) at the Mpl locus or containing one knockout and one Δ60 knockin allele (Mpl^−/Δ60). Values from at least 5 mice were determined. Note that in mice containing at least one Mpl wild-type allele or the Δ60 allele, platelet levels are only marginally affected by the presence of the Yall transgene, and reduced by approximately 70% compared with Yall;Mpl^−/− mice. (B) Schematic representation of the Mpl gene and the Δ60 knockin. Numbered boxes represent exons. A line connecting exon 8 with exon 11 marks the splicing event that generates the dominant-negative variant Mpl-tr. In the Δ60 allele, exons 10, 11, and a truncated version of exon 12 are inserted as a cDNA fragment, followed by an SV40-derived polyadenylation signal. Consequently, Mpl-tr cannot be formed from this allele.