On the other hand, mice of the same haplotype that were deficient in expression of MHC class II (C57BL/6 × 129/J Ab 0 ) developed not only extensive demyelination but also severe neurological impairment, and were, in fact, frequently moribund by 120 days postinfection (Njenga et al., 1996; Rivera-Quinones et al., 1998) .