library_construction_protocol: Genomic DNA (gDNA) was isolated from purified CD19+ pro-B cells. The V(D)J-seq protocol using gDNA was performed while omitting the negative depletion step and using custom JH primers. GDNA was sonicated to a range of 500 bp to 1000 bp using a Covaris S2 (Covaris).  Library barcoding was carried out using NEBNext Multiplex Oligos for Illumina (E7600S).