Regulates COOLAIR, a set of antisense transcripts originating from the 3' end of FLOWERING LOCUS C (FLC). Associates with single-stranded DNA that is part of an RNA-DNA hybrid, or R-loop, that covers the COOLAIR promoter. R-loop stabilization mediated by NDX inhibits COOLAIR transcription, which in turn modifies FLC expression. Expressed predominantly in dividing tissues such as young leaves, root tips, flower buds and embryos.