Fluorescent C bands of human chromosomes with 33 258 Hoechst stain

Air-dried preparations of human metaphase chromosomes normally exhibit a Q band fluorescence pattern with 33 258 Hoechst stain while the C band regions of 1,9,16, and most acrocentric short arms appear dull. If these stained and mounted slides are stored at room temperature in the dark for several days, a spontaneous change from C-negative to C-positive band some times occurs. We postulate that the pH of the buffered saline mounting medium during storage of the slides causes the C band shift since the results can be duplicated experimentally by lowering the pH of the mounting buffer from 5.5 to 4.0.