PTPRK mRNA is frequently underexpressed in NKTCLs. Gel images show the PTPRK mRNA expression levels as detected by semiquantitative RT-PCR in 2 samples of normal NK cells, 5 NKTCL cell lines (A), and 27 NKTCL primary tumors (B). The mRNA expression in each sample was normalized to β-actin expression. PTPRK mRNA was highly expressed in normal NK cells, but frequently not detected in NKTCL cell lines and significantly underexpressed in more than half of the NKTCL primary tumors.