The purpose of this RFA is to develop C57BL/6 ES cell lines that are efficient for high-throughput gene targeting and in successfully generating mice that can transmit the targeted mutation through the germline, both steps being necessary to create knockout mice on a C57BL/6 background. This RFA is a companion to two other RFAs (HG-05-007 and HG-05-008), all three of which are part of the NIH Knockout Mouse Project (KOMP).