An ultra-conserved poison exon in the Tra2b gene encoding a splicing activator is essential for male fertility and meiotic cell division. - Dalgliesh C; Aldalaqan S; Atallah C; Best A; Scott E; Ehrmann I; Merces G; Mannion J; Badurova B; Sandher R; Illing Y; Wirth B; et al. (The EMBO journal, 2025)