Earlier studies had found that proteins produced by genes in the MHC locus play a role in immunity and are involved in synaptic pruning during normal development. This raised a critical question: What exactly is the role of the protein product produced by the C4-A gene? To answer the question, scientists bred mice without the gene. They observed less-than-normal synaptic pruning in these mice, indicating that the protein’s role is to promote pruning and suggesting that too much of the protein leads to excessive pruning. In studies of these mice McCarroll, Stevens, Sekar, and their colleagues
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