Updated project metadata. Mammalian STE20-like protein kinases 3 and 4 (MST3/4) have recently emerged as crucial regulators of several key cellular processes such as proliferation, migration, and polarity. However, the downstream target pathways which are regulated by the kinase activity of MST3/4 remain poorly characterised. To assess downstream targets of MST3/4 kinases in an unbiased manner, we transiently co-depleted MST3 and MST4 in HeLa cells by siRNA and quantified the change in their phospho-proteome using SILAC.