E3 ubiquitin-protein ligase that mediates ubiquitination and subsequent proteasomal degradation of NUMB. E3 ubiquitin ligases accept ubiquitin from an E2 ubiquitin-conjugating enzyme in the form of a thioester and then directly transfers the ubiquitin to targeted substrates. Mediates ubiquitination of isoform p66 and isoform p72 of NUMB, but not that of isoform p71 or isoform p65. . Isoform 2 provides an endocytic scaffold for IGSF5/JAM4. . This is the function of LNX1 (ligand of numb-protein X 1, Ensembl gene identifier ENSG00000072201).