Birth defects in children often present with varying degrees of severity, even though the underlying causative genetic mutations may appear to be identical. The Grainyhead-like (Grhl) transcription factor family, in particular family members Grhl2 and Grhl3, are critical regulators of embryogenesis, and loss of these genes in both humans and animal models leads to congenital disorders and embryonic lethality. Our work has shown that genetic models lacking Grhl2 (mouse) and grhl3 (zebrafish) present with partially penetrant and variable phenotypes, despite a single engineered mutation in each. We have therefore focussed on understanding the genetic mechanisms responsible for these phenotypes. We have shown that the mouse Grhl2-/- phenotype (which comprises failed neural tube closure and embryonic lethality) can be partially rescued through genetic reduction of the soluble morphogen Noggin (de Vries, Development, 2024), an antagonist of the Bone Morphogenetic Protein (BMP) pathways; we are currently undertaking further transcriptomic analyses to understand this rescue, and how we can drive rescue of tissue fusion even further. We have also uncovered a novel enhancer eleemtn thatdrives craniofacial expression of Grhl2, and leads to cleft palate when mutated (de Vries, Dev Biol, 2021). Additionally, we have also shown that grhl3-/- zebrafish embryos present with variably severe phenotypes in axial and craniofacial development (Miles, Sci Rep, 2017; Mathiyalagan, Dev Dyn, 2025). Through interrogation and modulation of both genetic (transcriptional) and environmental (housing) conditions, we have begun to build up a profile of molecular and epigenetic factors that underpin phenotypic severity and pleiotropy within the Grhl-dependent pathways. This foundational knowledge will be critical to understanding the underlying genetic and epigenetic signatures that underpin the severity of incidence of multiple human birth defects.