The mouse 3D8 anti-DNA antibody can enter cells and localize in the cytoplasm, primarily facilitated by the complementarity-determining region 1 of the variable light chain (CDR L1) domain. In this study, we grafted the CDR L1 loop from 3D8 onto non-cell-penetrating IgG antibodies to investigate whether these IgGs could acquire cytoplasmic localization ability while retaining antigen-binding activity. One of three IgGs was successfully delivered into the cytoplasm while maintaining antigen-binding activity. In silico protein modeling suggests that this capability is linked to structural similarity between CDR L1 in the grafted Ab and that in 3D8. This study proposes a strategy to confer cell-penetrating capability by incorporating a specific CDR loop into an antibody backbone while retaining affinity.