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stowaways in the 1500s. During a breeding craze in the nineteenth
century that also gave us most of today’s breeds of dogs and cats,
amateur rodent enthusiasts bred “fancy” strains from wild mice and
rats, usually selecting for coat color. Early in the twentieth century,
a Harvard graduate student named Clarence Cook Little obtained a
pair of mice with well-documented bloodlines and began breeding
them for his research on genetics.^18 Mice reproduce quickly, reach-
ing sexual maturity around six weeks of age. They have litters of
at least four and as many as ten pups, and they can easily produce
a dozen litters a year. Their reproductive capacity made them the
species of choice for students of mammalian genetics. Little bred
brother to sister for over twenty generations and selected the most
vigorous offspring. By using the inbreeding techniques already
employed by fanciers, he engineered strains that frequently devel-
oped cancerous mammary tumors. At the same time, Little and
others strove to defi ne cancer in genetic terms.^19 As Karen Rader
explains, “The problem thus defi ned, inbred mice were the required
standard” for cancer research.^20 The mice developed tumors, bred
quickly enough for researchers to observe the disease’s generational
course, and, through inbreeding, had the stable genetic material
that eliminated unwanted variability. Most important, because
mice are mammals, scientists presumed that diseases in mice fol-
low courses similar to those that affl ict human beings. Mice became
a morally acceptable stand-in for humans in medical research. Con-
sequently, researchers created numerous distinct strains of mice for
particular research applications. These include nude (or hairless)
mice, which lack a thymus and thus do not reject implanted tumors
from other species, such as humans. Severe combined immune defi -
cient mice are a “souped-up” version of the nude mouse, valued for
studies of immunodefi ciency.
The growth of research in molecular biology during the 1970s
and 1980s created even greater demand for and supply of mice
because of the relative ease with which their genes can be modi-
fi ed at the molecular level.^21 The “knockout” mouse has a partic-
ular gene inactivated, demonstrating the behavior, appearance, or
biology of an individual who lacks that gene. Genes from other