The characteristic staining pattern of PCNA and TK-1 seen in both FL and DLBCL with 3+ or 4+ in proliferating (i.e. MIB-1 positive) cells and 1+ to 2+ staining of quiescent cells is explained by the fact that both PCNA and TK-1 show a striking increase in the expression during S-phase but are also expressed albeit to a lesser extent in other phases of the cell cycle[13,18]. This evidence concerns the gene TK1 and diffuse large B-cell lymphoma.