Northern blot analysis of (A) c-fos and (B) GAPDH mRNA in Jurkat cells. RNA was extracted from cells serum-starved (1% FCS) for 24 hours (lane 1) and then treated for 30 minutes with 15% FCS (lane 2), 15% FCS plus recombinant p24 (lane 3), 15% FCS plus recombinant Tat preincubated with an anti-Tat MoAb (lane 4), and 15% FCS plus recombinant Tat (lane 5).