PTK7 and glioblastoma: The few reported proteins identified through individual aptamer probes include pigpen from the rat endothelial cell line YPEN-1 [15], Tenascin-C of U251 glioblastoma cells [16], immunoglobulin heavy mu chain in Burkitt’s lymphoma cells [17] and protein tyrosine kinase-7 (PTK7) on CCRF-CEM T-cell acute lymphoblastic leukemic cells [18].