Terminal deficiencies at the tip of the X chromosome can be induced at a high frequency (0.2-0.3%) by irradiating Drosophila females carrying a homozygous mutator (mu-2) with low doses of X-rays. These terminal deficiencies are unstable, since over a period of 3 1/2 years DNA sequences were lost from their distal ends at a rate of 75 bp per generation, presumably due to the absence of a complete wild-type telomeric structure. Breakpoints of these deletions in the 5' upstream regulatory region of the yellow gene, giving rise to a mosaic cuticle pigmentation pattern typical of the y2 type, were used to define the location of tissue-specific cis-acting regulatory elements that are required for body, wing or bristle pigmentation.