Use of a DNA or RNA probe to detect the presence of the complementary DNA sequence in cloned bacterial or cultured eukaryotic cells. Also used for locating genes on chromosomes. The process is: Prepare microscope slide with cells in metaphase of mitosis. Treat slide with a weak base. Thus denaturing the DNA. Pour radioactively labeled probe onto the slide. Expose slide to photographic emulsion for a few days or weeks. Develop emulsion.