Decision trees for managing genetically burdened laboratory animal lines under Directive 2010/63/EU

The breeding and maintenance of genetically modified laboratory animal lines with a harmful phenotype (hereafter referred to as "genetically burdened lines")-that is, lines in which pain, suffering, distress, or lasting harm may arise as an inherent consequence of the genotype-pose specific ethical and regulatory challenges for animal facilities and researchers. Within the European Union, these challenges are framed by Directive 2010/63/EU, which subjects genetically modified animals with harmful phenotypes and related breeding activities to project authorization and prospective severity assessment. Institutions responsible for such lines face recurrent practical questions: When does breeding constitute a regulated "procedure" requiring prior authorization? How should killing, tissue collection, and reuse be documented in local records and annual reports? In accordance with EU terminology, "continued use" refers to a single use consisting of multiple steps, whereas "reuse" describes the use of an animal in a new procedure after a previous one has been completed. And how should documentation differ between burdened and non-burdened phenotypes arising within the same colony? Here, we present decision trees that provide a structured and reproducible framework to support consistent decision-making for genetically burdened lines. The trees are illustrated using the German implementation of Directive 2010/63/EU (the German Animal Welfare Act, GAWA, and the German Laboratory Animal Welfare Ordinance, GLAWO) as a working example; however, the underlying decision logic is jurisdiction-neutral and could be adapted to other European and international regulatory systems by mapping national terminology to equivalent categories.