Table 1.

Comparison of 8G7G3/1 and SP141 TTF-1 MAbs

Tissue	8G7G3/1 MAb: n (%)	SP141 MAb: n (%)
Normal lung—alveolar epithelium	7/7	7/7
Adenocarcinoma of the lung	35/35	35/35
Atypical squamous lesions—12 SCCs+1 case of bronchial squamous dysplasia	0/13	5+1/12+1 (∼46%)
Pleomorphic/sarcomatoid carcinoma of the lung (spindle-cell component only)	2/12 (17%)	4/12 (∼33%)
Epithelioid malignant mesothelioma	0/66	0/66
Sarcomatoid mesothelioma	0/19	8/19 (∼42%)

We are unaware of any systematic data on the actual levels of TTF-1 expression in, for example, sarcomatoid carcinomas of the lung and sarcomatoid mesotheliomas, for estimation of the rates of true/false positive/negative IHC labelling of these tumours. Existing evidence indicates that positive IHC labelling for TTF-1 protein may not represent ‘false’-positive labelling, but rather ‘true’ labelling in these cancers (ie, detectable labelling represents high affinity of the SP141 MAb in particular for low levels of TTF-1 protein in different cancers, with loss of diagnostic specificity for tumour classification by IHC). Ideally, these assessments should be made in comparison with true zero/augmented levels of mRNA as the template for TTF-1 protein—and as such the ‘gold standard’ test—but to the best of our knowledge, systematic data on mRNA levels in these sarcomatoid tumours are unavailable (see text).

IHC, immunohistochemical; MAb, monoclonal antibody; SCC, squamous cell carcinoma; TTF-1, thyroid transcription factor-1.