Gene testing involves examining a person's DNA - taken from cells in a sample of blood or, occasionally, from other body fluids or tissues - for some anomaly that flags a disease or disorder. The DNA change can be relatively large: a missing or added piece of a chromosome - even an entire chromosome - that is visible under a microscope. Or it can be extremely small, as little as one extra, missing, or altered chemical base. Genes can be overexpressed (too many copies), inactivated, or lost altogether. Sometimes, pieces of chromosomes become switched, or transposed, so that a gene ends up in a location where it is permanently and inappropriately turned on or off.
In addition to studying chromosomes or genes, genetic testing in a broader sense includes biochemical tests for the presence or absence of key proteins that signal aberrant genes.