The function of WDR91 (WD repeat domain 91, ENSG00000105875) is as follows. Functions as a negative regulator of the PI3 kinase/PI3K activity associated with endosomal membranes via BECN1, a core subunit of the PI3K complex. By modifying the phosphatidylinositol 3- phosphate/PtdInsP3 content of endosomal membranes may regulate endosome fusion, recycling, sorting and early to late endosome transport (PubMed:26783301). It is for instance, required for the delivery of cargos like BST2/tetherin from early to late endosome and thereby participates indirectly to their degradation by the lysosome (PubMed:27126989). May play a role in meiosis (By similarity). {ECO:0000250|UniProtKB:Q7TMQ7, ECO:0000269|PubMed:26783301, ECO:0000269|PubMed:27126989}.