Mice are in many ways similar to Homo sapiens on a fundamental level. That is why the law in this part of the world only permits scientists to conduct research on human embryonic stem cells when they have “clarified in advance” their specific questions by using animal cells as far as possible. However, such tests are often pointless — and sometimes even misleading, as a recent study demonstrates.

In a great majority of cases, the Y chromosome determines sex in mammals. The African pygmy mouse M. minutoides is an exception to this rule. In this species, which is a close relative of the house mouse, it is the X chromosome that determines sex. Researchers in France have just identified this unexpected case of sex determination. These scientists have demonstrated a particular chromosomal rearrangement on the X chromosome of this mouse. This work should provide a clearer understanding of how classic sex determination functions in mammals.

Some mouse sperm can discriminate between its brethren and competing sperm from other males, clustering with its closest relatives to swim faster in the race to the egg. But this sort of cooperation appears to be present only in certain promiscuous species, where it affords an individual’s sperm a competitive advantage over that of other males.

New information about the genes involved in a mammal’s early brain development, including those that contribute to neurological disorders such as autism and mental retardation, has been discovered. The study is the first to use high-throughput sequencing to uncover active genes in developing brains, and it may be the best evidence yet for the activity in the brain of a large number of genes.

Scientists have announced the creation of the first genetically modified mammals developed using zinc finger nuclease technology.