/Mob4 is essential for viability and male fertility in Drosophila melanogaster. The Mob4P and Mob4SVC alleles show 3rd instar lethality when homozygous. Ubiquitous depletion of Mob4 via RNAi results in complete larval/pupal lethality. Male-specific depletion of Mob4 in the germline leads to sterility, while female germline depletion has no effect. Human Mob4 rescues all Mob4 RNAi phenotypes, confirming conserved function.