The use of mouse genetic models requires an efficient system of unique
animal identifiers. The means that tissue must be obtained from
each animal for genotyping and that each animal must be tagged so it
can be quickly distinguished from other animals. A
variety of methods have been employed to tag mice including ear tags,
ear punches, tatoos and subcutateous chips. None of these
approaches provide tissue so they must be coupled with a tissue biopsy,
usualy a piece of tail tip,
for genotyping. Alternatively, toe biopsy approach simultaneously
provides tissue and a recognizable identifier to distinguish
animals. Toe biopsies have other advantages, namely they do not
chronic foreign body irritation, have less infection potential, can not
fall off and require no
specialized equipment.(1-6) The potential side effects of toe
biopsies are similar to that of tail biopsies and are minimal if
animals are biopsied in the recommended age range, and do not have
health problems at the time of the procedure such as skin diseases,
impaired immunity or thrombocytopenia. Like all live animal
identification requires proper training and prior IACUC
approval. For genotyping procedures that are ammenable to a
minimal amount of input material, cheek swabs are a less invasive
alternative to toe or tail biopsies, but do need to be coupled with a
permanent tagging procedure.(7)
Mouse pups typically spend 3 weeks with their mothers before
being weaned. During the first week they are too small for long
term identifcation methods. Between 7 and 10 days of age
their toes become well separated and can be biopsied with small
dissecting scissors. At this age the tissue is still soft and
the toe biopsy procedure appears to cause minimal distress. A 7
day old pup, for example, will typically object more
to being picked up and separated from its littermates than it will to
having its tail or toes
biopsied. At older ages, mouse pups become more alert and the toe
tissue becomes firmer from ossification. It is preferable to use
younger animals because older animals require additional steps are
taken to provide analgesia (e.g. with inhaled isoflourane) and ensure
hemostasis. The use of anesthesitics significantly increases the
risk of the procedure so biopsies should not routinely be performed on
1. The operator should wear gloves and protective clothing as per
2. Pick up a mouse pup by the scruff of the neck. When holding a
mouse do not squeeze the animal so hard that its breathing is
impaired. Do not hold it too loosely or it will squirm and become
Taking care to hold a mouse properly
is the most important aspect of ensuring its comfort and safety.
3. With a sharp pair of dissecting scissors remove a mouse toe and let
the tissue fall into a 1.5 mL microcentrifuge tube. If more than
one toe is being collected then pool them into the same tube. It is
important dissect the tissue proximal to the interphalangeal
joints so it can easily be recognized in the future.
4. Refer to the toe numbering scheme
identify the animals with a unique ID. The system uses at most
two toes from each foot and provides 0 - 9999 unique IDs. In
practice smaller numbers can be used as long as care is taken not to
add that two mice with the same toe IDs to the same cage. Cages
cards and inventory records should include the complete unique
identifier for each.
5. Place the pups which have been identified into a new cage to
distinguish them from their littermates. When all of the pups have been
identified, return the pups to their mothers cage.
6. Prepare DNA from the tissues according to the Tail
7. Enter the new Pup IDs into the murine
(MPD) along with genotype information when it is
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carcinomas of the skin at ear tag sites in aged FVB/N mice. Comp Med.
2. Cover CE, Keenan CM, Bettinger GE. Ear tag induced Staphylococcus
infection in mice. Lab Anim.
3. Waalkes MP, Rehm S, Kasprzak KS, Issaq HJ. Inflammatory,
proliferative, and neoplastic lesions at the site of metallic
identification ear tags in Wistar [Crl:(WI)BR] rats. Cancer Res.
1987 May 1;47(9):2445-50.
4. Le Calvez S, Perron-Lepage MF, Burnett R. Subcutaneous
microchip-associated tumours in B6C3F1 mice: a retrospective study to
attempt to determine their histogenesis. Exp Toxicol Pathol.
5. Nadon NL, Draeger K. Genomic DNA analysis from mouse toe
lysates. Transgenic Res.
6. Vachon P. Anatomical and histological observations of fore- and hind
limb toes in adult mice after amputations performed at the age of two
weeks. Can J Vet Res.
7. Meldgaard M, Bollen PJ, Finsen B. Non-invasive method for sampling
and extraction of mouse DNA for PCR. Lab