STK40 is a pseudokinase that functions as a substrate adaptor for the CRL4COP1/DET1 E3 ubiquitin ligase complex, facilitating targeted protein degradation through K48-linked polyubiquitination 12. The protein lacks kinase activity due to substitutions of conserved residues that prevent ATP binding, but retains a functional C-terminal sequence that enables binding to COP1 1. STK40 regulates multiple cellular processes by promoting proteasomal degradation of key transcription factors including FOXO1/4, c-Jun, and P57Kip2 234. In T cell biology, STK40 promotes pathological TH1/TH17 differentiation and limits T cell exhaustion during chr1 viral infection by controlling AP-1 signaling 23. The protein also regulates trophoblast fusion in placental development and has been implicated in various disease contexts including autoimmune diseases, preeclampsia, psoriasis, and cancer 456. STK40 can modulate NF-κB and JNK/MAPK signaling pathways and may serve as a negative regulator of YAP/TAZ in the Hippo pathway 56. These diverse functions position STK40 as an important regulatory hub controlling cell fate decisions through post-translational mechanisms.