Current technology can be used to knock out specific genes in the offspring of mice, to knock in genes with specific mutations, to selectively silence or express gene products in certain tissues, and, if desirable, to do all of these on demand.1 It has made Mus musculus a favourite tool in the modern biomedical research laboratory.

The idea that the mouse is a researcher’s aide is not a new one. More than a century ago Dr William Welch, one of the four founding “fathers” of The Johns Hopkins Hospital and Chief of Pathology, directed his trainee to the mouse laboratory for “broadening... knowledge of pathology as well as deepening it...”.2 Today, many pathologists have become intimately familiar with mouse pathology, but the tides have changed; these days the mouse is brought by the researcher to the pathologist to determine how closely the changes in that genetically engineered mouse mimic human disease.

Disruption in bone morphogenetic protein (BMP) signalling in genetically engineered mice led to insights into the human …