The function of STK38 (serine/threonine kinase 38, Ensembl gene identifier ENSG00000112079) is as follows. Serine/threonine-protein kinase that acts as a negative regulator of MAP3K1/2 signaling (PubMed:12493777, PubMed:15197186, PubMed:17906693, PubMed:7761441). Converts MAP3K2 from its phosphorylated form to its non-phosphorylated form and inhibits autophosphorylation of MAP3K2 (PubMed:12493777, PubMed:15197186, PubMed:17906693, PubMed:7761441). Acts as an ufmylation 'reader' in a kinase-independent manner: specifically recognizes and binds mono- ufmylated histone H4 in response to DNA damage, promoting the recruitment of SUV39H1 to the double-strand breaks, resulting in ATM activation (PubMed:32537488). {ECO:0000269|PubMed:12493777, ECO:0000269|PubMed:15197186, ECO:0000269|PubMed:17906693, ECO:0000269|PubMed:32537488, ECO:0000269|PubMed:7761441}.