The NOD mouse has been a critical tool in the quest to understand the genetic control of type 1 diabetes (T1D), and over 25 murine insulin-dependent diabetes (Idd) loci that modulate the natural history of T1D have been identified (1). Several of the candidate genes identified in NOD mice also play a role in human T1D, suggesting that dysregulated immune pathways in NOD may closely resemble those found in humans, and justifying continuing work on the genetic origin of T1D in NOD (2). The role of specific genes in T1D has been explored by constructing congenic mice (carrying disease-protective Idd loci), by creating transgenic mice, and by knocking out genes. In each case, however, these studies have had a significant potential drawback: the presence of “passenger DNA” that can potentially confound the interpretation of the results (Fig. 1A, top panel) (3). Knockout mice, for example, have previously been made almost exclusively using non-NOD embryonic stem (ES) cells; the resulting non-NOD mice were backcrossed to NOD mice. The backcrossing process ensures that non-NOD genetic material is bred along with the genetic region of interest (Fig. 1A, top panel). This is not merely a theoretical concern, as some published studies of transgenic or deleted genes showed an effect on T1D that was later proven to arise from non-NOD passenger DNA (3).
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