Methylation of exon 1 of p15/INK4b (bottom panel) and p16/INK4a (top panel) in representative BL biopsies, BL lines, LCLs, and normal B cells. Genomic DNA was digested with HpaII (H),Msp I (M), or left undigested (U), and analyzed by PCR followed by hybridization with an internal probe, as shown in Fig 1.