Introduction: The malignant transformation leading to a maturation arrest in B-cell precursor acute lymphoblastic leukemia (BCP-ALL) occurs early in B-cell development, in a pro-B or pre-B cell, when somatic recombination of variable (V), diversity (D), and joining (J) segment immunoglobulin (IG) genes and the B-cell rescue mechanism of V H replacement might be ongoing or fully active, driving clonal evolution. In this study of newly diagnosed BCP-ALL, we sought to understand the mechanistic details of oligoclonal composition of the leukemia at diagnosis, clonal evolution during follow-up, and clonal distribution in different hematopoietic compartments., Methods: Utilizing high-throughput sequencing assays and bespoke bioinformatics we identified BCP-ALL-derived clonally-related IGH sequences by their shared 'DNJ-stem'., Results: We introduce the concept of 'marker DNJ-stem' to cover the entirety of, even lowly abundant, clonally-related family members. In a cohort of 280 adult patients with BCP-ALL, IGH clonal evolution at diagnosis was identified in one-third of patients. The phenomenon was linked to contemporaneous recombinant and editing activity driven by aberrant ongoing D H /V H -DJ H recombination and V H replacement, and we share insights and examples for both. Furthermore, in a subset of 167 patients with molecular subtype allocation, high prevalence and high degree of clonal evolution driven by ongoing D H /V H -DJ H recombination were associated with the presence of KMT2A gene rearrangements, while V H replacements occurred more frequently in Ph-like and DUX4 BCP-ALL. Analysis of 46 matched diagnostic bone marrow and peripheral blood samples showed a comparable clonal and clonotypic distribution in both hematopoietic compartments, but the clonotypic composition markedly changed in longitudinal follow-up analysis in select cases. Thus, finally, we present cases where the specific dynamics of clonal evolution have implications for both the initial marker identification and the MRD monitoring in follow-up samples., Discussion: Consequently, we suggest to follow the marker DNJ-stem (capturing all family members) rather than specific clonotypes as the MRD target, as well as to follow both VDJ H and DJ H family members since their respective kinetics are not always parallel. Our study further highlights the intricacy, importance, and present and future challenges of IGH clonal evolution in BCP-ALL., Competing Interests: MB received grants and personal fees from Amgen advisory board, speakers bureau, travel support, and personal fees from Becton Dickinson, Janssen, Pfizer speakers bureau and Jazz travel support, all outside the submitted work. MS and MK received a personal fee from Amgen speakers bureau, outside the submitted work. SS has received personal fees advisory board from AMGEN, Gilead Sciences, Pfizer, SERB SAS, honoraria speaker bureau from Akademie für Infektionsmedizin e.V., AMGEN, AVIR Pharma, CSi Hamburg GmbH, Gilead, Labor28, Novartis, Persberg Group GmbH/DGIM e.V., Pfizer, Vivantes GmbH, research grants from Protherics Medicines Development Ltd, and travel support from Gilead Sciences, Novartis, outside the submitted work. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2023 Darzentas, Szczepanowski, Kotrová, Hartmann, Beder, Gökbuget, Schwartz, Bastian, Baldus, Pál, Darzentas and Brüggemann.)