Purpose: Although English Language Arts (ELA) teachers have historically been expected to take the lead in literacy training, the domain of ELA has yet come to terms with what holds it together as a discipline. Within this conundrum, the author studied one group of ELA teacher leaders who led a professional development (PD) aimed at training teachers in disciplinary writing instruction. This study aims to explore the differences in perspectives between what constitutes disciplinarity for ELA teachers and teachers in other content areas. Design/methodology/approach: Over the course of two years, the author observed the PD, taking extensive field notes, collecting artifacts and conducting interviews. The author engaged in constant comparative analysis of the data throughout this time, open coding within each data source and then triangulating the data to support the author's finding. Findings: Whereas the ELA teacher leaders seemed to focus on general aspects of writing, teachers from the other content areas shared discipline-specific understandings about writing. The teachers and teacher leaders, however, did not explicitly discuss these differences in how they conceptualized writing instruction; rather, this tension was revealed through the author's analysis of the data. Originality/value: The findings of this study illustrate how a vague definition of writing in English and of disciplinary literacy has come to bear on one PD of writing. This study recommends future research to continue to develop clear epistemologies, purposes and literate practices of the disciplines related to ELA.