Metaphase spreads from male and female mice of various inbred and random-bred types were stained with the DNA-binding fluorochrome quinacrine mustard. This treatment caused the chromosomes to display cross-striation patterns of bright and dark bands visible with the fluorescence microscope. The intensity distribution of fluorescence along each chromosome was found to be distinctive and reproducible, so that it was possible to differentiate and identify all chromosome pairs in the mouse karyotype. On the average, the X chromosome was the fourth largest pair in female cells. The Y was usually the smallest chromosome and showed the strongest fluorescence. Densitometer tracing of fluorescent chromosomes resulted in distinctive density profiles which could assist in chromosome identification.
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