Most experiments necessitate the use of several animals simultaneously. Especially when the animals are randomized to different groups that receive different treatments, it becomes crucial to be able to recognize different animals at different time points all along the experiments. Just placing the animals in different cages alone or in groups with the cages numbered or posted otherwise is not a reliable enough procedure. It is always necessary to mark the individual animals in a reliable way. It is of utmost important that all researchers in the same laboratory or institution follow the same marking system to avoid confusion. Large amounts of work and animals may be lost if this issue is overlooked. Marking animals for later identification can be done with various methods as long as the researchers are familiar with the marking system, enabling the correct animals to be tracked until the end of the experiment. There are a number of commercially available instruments for this purpose.
Waterproof pens (permanent markers) in various colors are available in most stationers and bookstores. Marking of the tail is easy even in awake animals. The dye lasts for several days but not longer. The dye should be applied all around the tail, thickly and widely to assure future identification and to avoid misinterpretation. The color and the mark can easily be seen in albino animals, but may be more difficult to interpret in wild-type animals. We have not observed any inter-observer difficulty.