Single-cell PCR analysis of the housekeeping genec-raf-1 by seminested PCR analysis of HRS cells from lymph node involved by HD (lanes HD: 1 through 10). The amplified DNA was separated on 2% agarose/NuSieve gel electrophoresis and visualized by staining with ethidium bromide. Ø, molecular size markers; BL (blank), PCR amplification without DNA template to rule out contamination; P, total DNA extracted from the same lymph node involved by HD. C-raf-1 has been successfully amplified from 7 of 10 isolated single cells as demonstrated by the presence of a band of 180 bp in 7 of 10 micromanipulated single-cell DNA. This indicates the validity of our methodology for the single-cell isolation and the PCR amplification of the extracted DNA from the majority of HRS cells isolated from tissue sections of lymph node involved by HD.