The groups of Konstantinos Anastassiadis and Francis Stewart offer cutting edge technology in the construction of gene replacement vectors and bacterial artificial chromosomes (BACs) via homologous recombination (recombineering). Constructed vectors are then transfected into mouse or human ES cells and correctly manipulated ES cell clones are then used for further investigations. This technology allows for conditional loss of function studies and regulated overexpression in cells, mice and other animal models.