Lineage-based vaccine design is an attractive approach for eliciting broadly neutralizing antibodies (bNAbs) against HIV-1. However， most bNAb lineages studied to date have features indicative of unusual recombination and/or development. From an individual in the prospective RV217 cohort， we identified three lineages of bNAbs targeting the membrane-proximal external region (MPER) of the HIV-1 envelope. Antibodies RV217-VRC42.01， -VRC43.01， and -VRC46.01 used distinct modes of recognition and neutralized 96%， 62%， and 30%， respectively， of a 208-strain virus panel. All three lineages had modest levels of somatic hypermutation and normal antibody-loop lengths and were initiated by the founder virus MPER. The broadest lineage， VRC42， was similar to the known bNAb 4E10. A multimeric immunogen based on the founder MPER activated B cells bearing the unmutated common ancestor of VRC42， with modest maturation of early VRC42 intermediates imparting neutralization breadth. These features suggest that VRC42 may be a promising template for lineage-based vaccine design.