Nitrogen-responsive activator of Start; nuclear microprotein with transcriptional activator activity; bypasses Start arrest in a cln3 bck2 double mutant when overexpressed; interacts with subunits of the SBF complex (Swi4p-Swi6p); induced by nitrogen limitation and rapamycin with peak abundance at Start; proposed to promote the G1/S transition by rewiring the G1/S transcriptional machinery under poor nitrogen conditions upstream of the Whi5-inhibited form of SBF and independent of Cln3 and Bck2