The present invention relates to methods of profiling tumours and characterisation of the tissue types associated with the tumours. A gene expression profile is obtained from the tissue sample, the genes ranked in order of their relative expression levels and the tissue type identified by comparing the gene ranking obtained with a database of relative gene expression level rankings of different tissue types. This gives a means to identify primary tumours and to determine the identity of a tumour of unknown primary. The invention also provides a method of treatment of a tumour by diagnosis of primary tumours identified by the methods described.