Based on studies performed with a selected mouse monoclonal anti-transketolase- like (TKTL)-1 antibody (clone JFC12T10), overexpression of TKTL-1 has been shown to be correlated with poor survival and increased metastatic spread in several human tumor entities. Since the clinical aggressiveness mediated by TKTL-1 has been partially related to resistance to hypoxia,we originally aimed to explore the influence of hypoxia on the expression of TKTL-1. Unexpectedly, results of our experiments indicated that the antibody clone JFC12T10 lacks target specificity. Since the majority of data on the role of TKTL-1 in human cancer is based upon studies performed with this antibody clone, we subsequently re-evaluated the expression of TKTL-1 in six different cancer cell lines (HeLa, MCF-7, A549, HT-1080, M21 and TF-1). Using RT-PCR and consecutive sequence analysis, we show that transketolase (TKT), not TKTL-1, is the dominant isoform of transketolases in the cell lines analyzed. Our data argue against a major role of TKTL-1 for the metabolism of cancer cells.