The goal of the Transgenic Mouse Core Facility is to produce genetically modified mice for Cancer Center members at City of Hope. The facility performs microinjection of recombinant DNA into zygotes, electroporation of DNA into embryonic stem (ES) cells for gene targeting, and transfer of the modified ES cell genome into mice by chimera formation. The facility also assists investigators with the experimental design and maintains a bank of recombinant DNA vectors and genomic DNA libraries used commonly in the construction of gene targeting vectors. The facility also offers the following supplementary services: (1) ovum transfer to re-derive pathogen-carrying strains of mice, (2) cryopreservation of mouse strains by embryo freezing, and (3) maintaining specialized mouse strains useful in transgenic research. The availability of these services enables more efficient and economical use of genetically modified mice in research by removing (1) the significant investments of time and resources required for investigators to produce them on their own, and (2) the high expense of contracting with a company. During the calendar year 1999, five Cancer Center members and one who was not a member utilized the facility. There are currently ten Cancer Center members funded by peer-reviewed grants who are preparing to use the facility, and two Cancer Center members have requested letters from the facility as part of applications for peer-reviewed grants. Peer-reviewed usage of this core by Cancer Center members is 100% of the total usage.