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Office of Animal Welfare

Genotyping of Laboratory Mice

Purpose

To establish IACUC expectations and requirements regarding genotyping methods for laboratory mice.

Background

It is often necessary for researchers to genotype the offspring of genetically modified mice or to individually identify mice for scientific reasons.

Genotyping is most commonly accomplished by obtaining tissue samples from which DNA is isolated and analyzed (e.g., by PCR, Southern blot analysis). The most common tissue sampling methods used in mice are ear punch biopsy and tail transection (also referred to as tail biopsy, tail snip, tail tip amputation, or tail clip) but may also be accomplished via blood sampling or oral swabs.

Tail transection involves cutting a piece of tissue from the terminal end of the tail. In young rodents, the vertebral endplates continue to ossify after birth. Thus, this procedure is often performed without anesthesia when less than 3 mm of tissue is removed from very young animals. However, for most strains, vertebral ossification is complete around 21 days and tail transection of older animals (> 21 days of age) may be painful and is associated with a higher risk of complications.

Policy

Genotyping should be performed using the least invasive method available that will meet scientific needs. The IACUC considers ear punch biopsy to be less invasive than tail transection, and notes the added advantage that ear punches can be used for both identification and genotyping.

Less-commonly used methods for genotyping mice involve blood sampling and oral swabs to collect cells from the buccal mucosa. Blood sampling must be done in accordance with the IACUC Policy: Blood Collection in Research Animals. Blood sampling and buccal swab methods must be described within the approved protocol.

Ear biopsy: Anesthesia and analgesia are not required for the collection of small (e.g., 2 mm) ear punch samples in mice. In mice, the ear is usually sufficiently developed for ear punch biopsy after approximately 4 days of age.

Tail transection (for the purpose of genotyping) is permitted if IACUC approval is obtained, and with the following specifications:

  • Removal of ≤ 3 mm of tail tip in mice ≤ 21 days of age may be performed without anesthesia or analgesia
  • Tail transection in animals > 21 days of age must be performed under appropriate anesthesia (e.g., ketamine/xylazine, isoflurane, etc.) and a minimum of one dose of a systemic analgesic (e.g., buprenorphine, meloxicam, ketoprofen, carprofen) must be administered.
  • Removal of > 3mm of tail in any age mouse requires scientific justification and typically must be performed under appropriate anesthesia (e.g., ketamine/xylazine, isoflurane, etc.) and a minimum of one dose of a systemic analgesic (e.g., buprenorphine, meloxicam, ketoprofen, carprofen) must be administered.
  • If required, mice may undergo a second tail transection, but this must be at least one week after the first tail transection and anesthesia and analgesia must be used regardless of the age of the mice.
  • Researchers should be prepared to control bleeding as necessary following tail transection. Direct pressure is often adequate for small tail biopsies in weanling mice. For older animals, larger lengths, and second tail transections, other methods (e.g., cautery) may be necessary to achieve hemostasis.
  • Instruments used for tail transection must be well-maintained such that they are clean and sharp.

Deviation from the above conditions requires strong scientific justification and IACUC approval.

Anesthesia and Analgesia recommendations for rodents can be found in Appendix 1 of the IACUC Policy: Analgesia in Research Animals

Toe clipping for the purpose of genotyping or identification is highly discouraged and requires strong scientific justification and IACUC approval.

References

  1. Bonaparte, D. et al., 2013. FELASA Guidelines for the refinement of methods for genotyping genetically-modified rodents: a report of the Federation of European Laboratory Animal Science Associations Working Group. Laboratory Animals 2013:1-10.
  2. Hankenson, C. et al. 2008. Evaluation of tail biopsy collection in laboratory mice (Mus musculus): vertebral ossification, DNA quantity, and acute behavioral responses. JAALAS 47(6):10-18.
  3. Hankenson, C., Braden-Weiss G.C and J. Blendy. 2011. Behavioral and activity assessment of laboratory mice (Mus musculus) after tail biopsy under isoflurane anesthesia. JAALAS 50(5):686-694.
  4. Jones, C.P., Carver, S. and L.V. Kendall. 2012. Evaluation of common anesthetic and analgesic techniques for tail biopsy in mice. JAALAS 51(6):808-814.​

Approval/Review Dates

Originally A​​​pproved: 07/18/2013
Last Reviewed/Revised by the IACUC: 01/23/2025

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