Bifunctional enzyme that possesses both UDP-N- acetylglucosamine 2-epimerase and N-acetylmannosamine kinase activities, and serves as the initiator of the biosynthetic pathway leading to the production of N-acetylneuraminic acid (NeuAc), a critical precursor in the synthesis of sialic acids. By catalyzing this pivotal and rate-limiting step in sialic acid biosynthesis, this enzyme assumes a pivotal role in governing the regulation of cell surface sialylation, playing a role in embryonic angiogenesis (PubMed:10334995, PubMed:11326336, PubMed:14707127, PubMed:16503651, PubMed:2808337, PubMed:38237079). Sialic acids represent a category of negatively charged sugars that reside on the surface of cells as terminal components of glycoconjugates and mediate important functions in various cellular processes, including cell adhesion, signal transduction, and cellular recognition (PubMed:10334995, PubMed:14707127). {ECO:0000269|PubMed:10334995, ECO:0000269|PubMed:11326336, ECO:0000269|PubMed:14707127, ECO:0000269|PubMed:16503651, ECO:0000269|PubMed:2808337, ECO:0000269|PubMed:38237079}. This is the function of ENSG00000159921 (GNE, glucosamine (UDP-N-acetyl)-2-epimerase/N-acetylmannosamine kinase).