The progress of world-wide efforts in genome mapping and sequencing is uncovering a number of novel genes whose functions cannot be predicted from their structures, thereby demonstrating the need for the systematic collection of biological information other than their sequences to exploit fully the genomic data. Since one of the most fundamental pieces of information is the expression profile of individual genes, various approaches to genome-wide gene-expression scanning are being explored, based on differential hybridization, comparative cDNA sequencing and message-display techniques. Although each approach has unique advantages and drawbacks, the accumulation of expression data on individual genes is expected to shed light on the functions of genes that are revealed by genome analysis but whose function is not known. At the same time, further improvements in these techniques, as well as the development of novel methodologies, are required to explore fully the genome expression status in various biological situations.