Classroom interaction is a very complex phenomenon and an approach from multimodality enriches the understanding of the communicative landscape between teachers and learners. This perspective is essential to describe what happens in a classroom of deaf students and to understand the semiotic potential of the resources used to teach and learn when visual semiosis is the primary media for the exchange of meaning. This research is part of a larger study on literacy and semiotic mediation in Chilean school classrooms (Fondecyt 1130684) and aims to the multimodal description of genres, semiotic media and modes used by two teachers, one of Natural Sciences and the other of Social Sciences, interacting with a group of deaf students of 8th grade. A Multimodal Discourse Analysis was carried out upon the observation of a whole curricular unit. Among the findings, we highlight that both teachers structure their lessons mainly around one genre: definition (reports), co-deployed in face to face interaction by using sign language with the whiteboard, computer presentations, online software, notebooks and printed learning activities. In this case study, definitions work visually to represent both technical images and technical categories of the dominant language through continuous and discontinuous writing for these young deaf students to conceptualize the world from the visual logic of their first language.