Ubiquitin-like modifier activating enzyme 1 (UBA1) is an enzyme which in humans is encoded by the UBA1 gene. UBA1 participates in ubiquitination and the NEDD8 pathway for protein folding and degradation, among many other biological processes. This protein has been linked to X-linked spinal muscular atrophy type 2, neurodegenerative diseases, and cancers. The protein encoded by this gene catalyzes the first step in ubiquitin conjugation, or ubiquitination, to mark cellular proteins for degradation. Specifically, UBA1 catalyzes the ATP-dependent adenylation of ubiquitin, thereby forming a thioester bond between the two. It also continues to participate in subsequent steps of ubiquination as a Ub carrier. There are only two human ubiquitin-activating enzymes, UBA1 and UBA6, and thus UBA1 is largely responsible for protein ubiquitination in humans. Through its central role in ubiquitination, UBA1 has been linked to cell cycle regulation, endocytosis, signal transduction, apoptosis, DNA damage repair, and transcriptional regulation. Additionally, UBA1 helps regulate the NEDD8 pathway, thus implicating it in protein folding, as well as mitigating the depletion of ubiquitin levels during stress.