A widely perceived opportunity for the use of microarrays is the examination of gene expression across collections of cancer cells derived from both tumour cell lines and tumours. Primary goals of such a cataloging effort will be to determine whether there are subtypes of cancer that can be identified on the basis of identifiable patterns of gene expression and whether these types can be correlated with susceptibility of the cancer to particular treatment regimes. While pursuing these primary goals, it will also be possible to examine some longstanding, basic questions about expression profiles in cancer. Our presentation will examine a large set of expression profiles of melanoma cells. It will furnish analyses of the extents of heterogeneity and homogeneity between individual tumour specimens and of differences in the extent and types of expression between tumours and cell lines derived from them. We will also provide examples of the coordinated changes in gene expression that result from experimentally imposed alterations of the expression of genes showing interesting patterns of expression in melanoma.