PCR differential screening. mRNA from tumor samples of 3 patients with peripheral DLBLs (patients no. 19, 13, and 20) and 3 patients with PMBLs (patients no. 9, 10, and 12) were reverse transcribed and amplified by PCR using the T12GC anchor primer and the OPA 18 arbitrary primer. Amplified cDNAs were run side by side on a 6% sequencing gel. The arrow indicates the band seen only in amplified cDNAs corresponding to patients with PMBL.