Generation of a platelet-specific MYH9 knock-down mouse strain exhibiting myosin deficiency. (A) ES cells recombined with the floxed allele were injected into blastocysts to produce chimeric floxed mice (i). Deletion of the MYH9 exon 1 (MYH9Δ) was obtained by exposing the floxed allele to in vivo cre-recombinase expression (ii). (B) Western blot showing NMCH-IIA expression in several tissues from MYH9Δ mice (−/−) and control mice (+/+). An identical amount of protein lysate for control or MYH9Δ platelets was loaded in each lane, as shown by the similar levels of actin. (C, top panel) Western blot performed with increasing amounts of lysate from control (+/+) and MYH9Δ (−/−) platelets (numbers indicate the amount in μg of protein loaded on the gel), showing the residual myosin expressed in platelets (large gray arrow) and exon 1–deleted myosin (small black arrow). (Bottom panel) Western blot showing the regulatory light chain (RLC) expression in control and MYH9Δ platelets. (D) The entire population of platelets displayed decreased myosin content in MYH9Δ, as revealed by confocal immunofluorescence microscopy (1 image representative of 3 independent mice; bars, 2 μm).