A test for chromosomal changes in tumour DNA and the theory that some tumours arise from inactivation of both alleles for an anti-oncogene. Heterozygotes, who already have one mutation, are most vulnerable. The tumour (e.g. breast cancer) genome, (presumably homozygous) is compared with the constitutional (e.g. leucocyte) genome (presumably heterozygous) for changes that can be interpreted as a change from the heterozygous to a homozygous state. With an appropriate probe for a site near a suspected mutation, restriction fragment length polymorphism analysis is used to detect the change.