by the presence of a 5’-phosphorylation (Popow et al., 2011). In the second, 5’-phosphorylation by CLP1, a component of the TSEN complex in mammals, is required (Paushkin et al., 2004; Weitzer and Martinez, 2007; Zillmann et al., 1991), but the ligase that follows has not yet been discovered. Utilization of both pathways in human cells is documented, but their relative importance and functional redundancy in development and homeostasis remains unclear.
