To promote student writing development, integrated approaches such as genre-based writing instruction (GBWI) are advocated in tertiary education. However, most subject lecturers are not used to centralise writing in their subjects as they focus on content teaching. Capitalising on teacher learning within GBWI is therefore necessary. Design-based research can offer a fruitful learning environment for such innovative type of content and language integrated instruction. In a multiple case study (n=2) in Dutch higher professional education, we aimed to explore what subject lecturers can learn in a design-based research project in terms of scaffolding students' writing. Qualitative data on teacher learning were collected through logs and interviews before, during and after three GBWI interventions. These data were transcribed verbatim and analyzed using transcription software. Results showed the subject lecturers reported multifaceted learning outcomes, particularly concerning changed knowledge and beliefs. Some of these were directly related to GBWI (e.g., metalanguage, deconstruction, text features) whereas others were related to scaffolding language in subject learning more generally, and to the lecturers' teaching roles. Both lecturers also reported learning outcomes in terms of changed practices, but to a lesser extent. This may be related to the challenging character of enacting GBWI in the subjects. On a more general level, this study has yielded valuable insights into what factors are at stake when subject lecturers learn to enact GBWI. Further, it has shown the potential of a design-based research learning environment which we view as part of a causal field instigating subject lecturers' professional development.