RARA and acute promyelocytic leukemia: Indigo naturalis alone has no effect on APL, but indirubin in indigo naturalis could enhance the inhibition of arsenic disulfide in realgar on the proliferation and apoptosis of diffuse large celllymphoma cells and promote the transport of arsenic disulfide to APL cells, thus enhancing the degradation and treatment effect of PML-RAR α mediated by arsenic disulfide and accelerating the apoptosis of NB4 cells [56, 74].