Colony Planning

It's important to determine the number of breeding pairs you need to supply sufficient mice for your experiments. The Breeding Colony Size Planning Worksheet (pdf) is a very useful resource.

Mating combinations

  • Mating pair - one male, one female; common for strains with good fecundity.  Leave pair together continuously, whenever possible, to maximize production.
  • Mating trio - one male, two females; maximizes cage space for breeding and mothers assist in raising each other’s pups. Leave trio together continuously, whenever possible.
  • Male rotation - single male placed with 1-2 female(s) per week; useful to quickly establish a colony. Once male is removed, do not return until pups are weaned.

Reproductive characteristics

Mating age:  6-8 weeks of age
Gestation:  ~19-21 days
Wean age:  ~21 days; up to 28 days if preferred
Litter size:  2-12 pups; highly strain-dependent
Replace breeders:  ~7-8 months of age (several mutant strains have considerably shorter windows of optimal breeding performance)

How to select appropriate breeding schemes

Homozygous mutant (-/-) x homozygous mutant (-/-):

Heterozygous mutant (-/+) x homozygous mutant (-/-)

  • Can be used if only one gender is viable and fertile as a homozygote
  • Offspring:  50% homozygous; 50% heterozygous; genotyping needed
  • Controls: see Considerations for choosing controls

Heterozygous mutant (-/+) x heterozygous mutant (-/+)

  • Must be used if neither gender is viable or fertile as homozygotes
  • Offspring: 25% homozygous; 50% heterozygous; 25% wildtype; genotyping probably required
  • Controls:  see Considerations for choosing controls

Breeding scheme resources

Timed Pregnant Females

Setting up timed matings:

  • House stud males (individually) for 2 weeks prior to mating
  • Use 8-15 week old females
  • Add 1-2 female(s) into each stud male’s cage in the late afternoon, prior to the dark cycle
  • Check for vaginal plugs early the next morning (plug will dissolve over time)
  • The date a plug is observed is gestational day 0 to 0.5

NOTE:  Not all females will plug and not all plugged females will become pregnant; success rate varies by strain, male’s experience, conditions, etc.


Nagy A, Gertsenstein M, Vintersten K and Behringer R.  2003. Manipulating the Mouse Embryo:  A Laboratory Manual, 3rd ed.  Cold Spring Harbor Laboratory Press.  ISBN 0-87969-574-9.

Frequently asked questions

How many breeding pairs should I obtain to start my own colony?

A minimum of two to four breeding pairs are recommended for most strains; additional pairs are suggested for strains challenging to breed, or to expedite colony expansion.  Alternatively, it may be more cost-effective to have JAX provide study-ready mice for your experiments (see JAX® Breeding and Phenotyping Services).

What breeding information is available for JAX® Mice?

Consult the JAX® Mice datasheet for your strain of interest. Breeding performance is environment dependent; detailed baseline breeding statistics should be generated under your facility conditions.

My mice aren’t breeding well, what should I do?

See our Tips for poor breeders webpage.