The influence of 1,25(OH)₂D₃ on TM upregulation by ATRA. The various combinations of 1,25(OH)₂D₃ and ATRA were incubated with U937 cells for 24 hours. TM upregulation was expressed as increase of TM antigen levels in the cell lysate. Experiments were repeated independently three times. Values are means ± SD.