Copy and paste: Humanizing IgH 155 kb at a time.

Generating mice that express human antibodies is a lengthy and labor-intensive process. Knocking in human gene segments into mice has potential, but how large a gene segment can be knocked into a zygote? Nair et al. insert a 155-kb human VH locus directly into mouse zygotes by CRISPR-mediated homology-directed repair, generating functional humanized antibody mice in weeks rather than years.