Around a third of mammalian genes are essential for life, and the recent Nature paper from the IMPC ‘High-throughput discovery of novel developmental phenotypes‘  describes some achievements from sytematic study of these genes in knockout mice.
FURTHER EVIDENCE THAT MOUSE DATA IS RELEVANT TO CLINICAL STUDIES
Lethal genes in the mouse are known to be enriched for human disease genes [2,3]. When additional data from the IMPC was included on the genes essential for survival of the embryo, this enrichment was increased even further. More than half of the human disease genes considered were essential for mouse embryo survival. The study also found a remarkable correlation between the core essential genes in humans and mice.
Systematic knockout mouse screens provide data that could not be derived from human patients. These new results further underline the importance of mouse models in the study of human disease, and their relevance in a clinical setting.
INCOMPLETE PHENOTYPE PENETRANCE
A suprising observation from knockout mouse screens is the incomplete penetrance of phenotypes for many lines.
One example of this is the sub-viability of lines. The IMPC has found that in around 11% of knockout lines some homozygous pups were observed, but fewer than the 1 in 4 pups predicted by Mendelian genetics. Some pups were able to survive with the homozygous gene knockout, but some weren’t.
Incomplete penetrance is a result also echoed in DMDD data. For example, in the seven Adamts3 knockout embryos studied, all display subcutaneous edema and absent lymph sac, while only two display a bifid ureter.
Data from systematic screens of knockout mice is showing, on an unprecedented scale, that even for a complete gene knockout, the observed phenotypes can vary from embryo to embryo. Given the standardised background and allele construction, this is a suprising result and could suggest an underlying stochastic process.
CANDIDATES GENES FOR UNDIAGNOSED HUMAN DISEASE
As part of its systematic screen, the IMPC has identified 22 essential mouse genes with human orthologs that are not known to be associated with any human disease. These are potential candidates for undiagnosed diseases and could shine new light on the causes of genetic disorders.
Efforts are continuing to study knockouts of every gene in the mouse genome. As these datasets grow in size, so too does the potential for them to help us understand gene expression and the genetic basis of human disease.
The DMDD database of embryonic-lethal mouse knockouts can be found at dmdd.org.uk.
The IMPC database of knockout mice can be found at www.mousephenotype.org.
 The IMPC Collaboration (2016)
High-throughput discovery of novel developmental phenotypes
 B. Georgi1, B. F. Voight1, M. Bućan1 (2013)
From mouse to human: evolutionary genomics analysis of human orthologs of essential genes
PLoS Genet 9(5): e1003484. doi: 10.1371/journal.pgen.1003484
 J. E. Dickerson 1, A. Zhu1, D. L. Robertson1 K. E. Hentges1 (2011)
Defining the role of essential genes in human disease
PLoS ONE, 6(11), e27368. http://doi.org/10.1371/journal.pone.0027368