Creating a new color by omission of 3 end blocking step for simultaneous detection of different chromosomes in multi-PRINS technique.

In the multiple color primed in situ labeling (multi-PRINS) technique, the blocking step using ddNTPs, incorporated by a DNA polymerase, is an important procedure that blocks the free 3' end generated in the previous PRINS reaction, thus avoiding the next PRINS reaction using it as a primer to perform spurious elongation at non-desired sites. However, we found that omission of the blocking step never affected the correct identification of two chromosomes because the signals from the second PRINS reaction site always showed the pure original color. Nevertheless, taking advantage of the color mixing, we successfully used a multi-PRINS technique to create a third color using the two most common forms of labeled dUTP (biotin- and digoxigenin-labeled dUTP) and two fluorochromes (fluorescein and rhodamine) in order simultaneously to detect three chromosomes in the same cell. By arranging the labeling either in bio-dig-bio or in dig-bio-dig order in the sequential PRINS reaction, then detecting with a mixture of avidin-fluorescein/anti-dig-rhodamine or a mixture of anti-dig-fluorescein/avidin-rhodamine, the signals at the centromeres of three different chromosomes displayed perfect yellow, red and green colors, respectively. The entire procedure could be completed in less than 90 min because the blocking step was omitted. We showed that this is a practical and efficient way to carry out multiPRINS so that even more than three chromosome targets could be detected in the same cell.