Efficiency and specificity of Cre-mediated excision in Pf4-Cre × integrin β1 lox/lox mice. (A) Diagram of integrin β1 floxed and excised alleles. Numbered black boxes represent coding exons, loxP sites are depicted as gray triangles. A thick line indicates the position of the Southern probe. (B) Southern blot analysis of genomic DNA from mouse tissues digested with PvuII and hybridized to the probe shown above. A composite picture of 2 blots is shown. Whole indicates unfractionated bone marrow cells; MEG, highly pure megakaryocytes obtained from bone marrow by FACS sort and subsequent TPO culture; CD41⁻, bone marrow depleted of CD41⁺ cells using antibodies and magnetic beads; LEU, peripheral blood leukocytes; LIV, liver; SPL, spleen; KID, kidney; INT, intestine; OVA, ovary; THY, thymus; HEA, heart; LUN, lung; BRA, brain. (C) Cell-surface expression of integrin β1 protein on platelets (top) or megakaryocytes (bottom) from mice with the indicated genotypes was assessed by flow cytometry. Megakaryocytes obtained from bone marrow cell cultures grown in the presence of recombinant TPO were used. Note that integrin β1 protein expression is virtually absent on platelets and markedly reduced on megakaryocytes from Pf4-Cre × integrin β1 lox/lox mice.