In Northern blots, TAP1 mRNA levels in primary tonsillar B lymphocytes are reduced after 48 hours of incubation with B95.8 supernatant (lane 1, labeled B95.8) in comparison with the same B-cell preparation incubated with cell culture medium (lane 2, labeled RPMI) only. Immortalized B-cell lines established by EBV infection in vitro also show a reduced TAP1 expression, although the reduction was not as severe as in primary B lymphocytes incubated with B95.8 supernatant (lanes 3 through 7, labeled G.E., Be 3.2, Be 4.6, Be 9.1, and Be 9.2, respectively). The steady state of TAP2 mRNA is unaltered or only slightly affected in the same RNA preparations. GAPDH-specific hybridization of the Northern blot indicated that the amount of mRNA loaded is about the same in all lanes.