VWF levels in congenic mice. VWF plasma levels were determined. B6 represents C57BL/6J mice purchased from The Jackson Laboratory (n = 7). Mvwf5 BW mice were backcrossed to B6 to the N4 or N5 generation, and VWF levels on littermates that were BB (n = 18) or BW (n = 11) at both D6Mit329 and D6Mit339 were determined (see inset; numbers indicate centimorgan [cM] distances between markers and Vwf indicates the XhoI restriction fragment polymorphism described in “Methods”). *The difference between these 2 groups is statistically significant (P < .001 by t test). **The difference between these 2 groups is statistically significant (P < .004 by t test). There was no significant difference between the B6 and BB groups (P > .1 by t test). Error bars represent SD.