Experimental design and speed congenics used to shorten the period of backcrossing. (A) Percentage of the C57BL/6 polymorphism loci remaining in the backcrossed generation of mice. After 4 generations, a few mice had less than 3% of C57BL/6 background loci. (B) Experimental design for pX-induced RA in the Vegfr-1 tk^–/– background. *Confirmation of the incidence of arthritis in newly prepared BALB/c speed congenic pX and original BALB/c pX transgenic mice. **The incidence and degree of arthritis in these mice were examined. (C) Comparison of the incidence of arthritis between newly prepared BALB/c speed congenic pX transgenic mice and original BALB/c pX transgenic mice. No difference was observed during the first 3 to 6 months after birth.