Neuroendocrine differentiation of lung tumours is characterised by the 
expression of several neuroendocrine markers and is confined mostly to specific 
histological subtypes, i.e. small cell carcinomas and carcinoids. One of the 
markers seen in neuroendocrine tumours, high activity of the aromatic L-amino 
acid decarboxylase (AADC), is helpful in distinguishing the classic and variant 
small cell lung tumour subtypes. Here, we have analysed the expression and 
quantified the level of mRNA coding for AADC in human tumour cell lines by use 
of the reverse transcription and polymerase chain reaction (RT-PCR). High 
amounts of mRNA were detected in classic small cell lung carcinomas and a 
neuroblastoma cell line. Other cell lines (melanomas, non-small cell lung 
carcinomas and osteosarcoma) also showed AADC expression, but the levels were 
2-3 orders lower. Also, the tissue-specific (neuronal versus liver-specific) 
mRNA type has been estimated. Small cell lung carcinomas, neuroblastoma and 
melanoma expressed messenger RNA specific for neuronal tissues. Importantly, the 
non-small cell lung carcinoma cell lines expressed either liver-specific 
(non-neuronal) mRNA (cell line A549) or predominantly the neuronal (cell line 
NCI-H520) AADC message. These data indicate that a range of tumour cell lines 
transcribe the AADC gene and that two distinct types of AADC mRNA which reflect 
the embryonal (neuronal or non-neuronal) origin of the tumour may be produced in 
non-small cell lung cancer cells.