A method for detection of the tissue specificity of gene expression. Two tissues are compared: a deficient tissue from which single-stranded cDNA (driver cDNA) is prepared and attached to magnetic beads to which are also attached poly(T) sequences, and a normal tissue from which cDNA (tracer cDNA) is prepared. An excess of the driver cDNA hybridises with the tracer cDNA preparation from all except those mRNAs in which the first tissue is deficient, and will be removed by magnetic separation of the beads. The resulting cDNA can be used to create a subtraction library for examination of tissue-dependent expression.