Epigenetics
Genes are not all. And especially in mice.
Selected papers
Mehtali, M., LeMeur, M. and Lathe, R. (1990) The methylation-free status of a housekeeping transgene is lost at high copy-number. Gene 91, 179- 184
Dobie, K.W., Mehtali, M., McClenaghan, M. and Lathe, R. (1997) Variegated gene expression in mice. Trends Genet. 13, 127–130
Lathe, R. (2004) The individuality of mice. Genes Brain Behavior 3, 317–327