Overall effects of ATRA on RAR-mediated signaling. The data stress the diversity of mechanisms (basal repression, ligand-induced repression or activation, RAR degradation, cross talk with other key pathways) as well as important differences in ATRA levels implicated in these effects. In the hematopoietic lineage, RARA seems primarily involved in granulocytic differentiation, whereas RARG plays important roles in the biology of HSC and theirs niches.