SPI1 and acute myeloid leukemia: Accordingly, treatment with ATRA promoted upregulation of PU.1 (and PTEN) both in PML/RARA-positive NB4 cells (Figure 2e) and in the PML/RARA-negative OCI-AML2 and HL-60 AML cell lines (Figure 2f), definitely suggesting the existence of PML/RARA-dependent and -independent mechanisms by which ATRA can induce PU.1 transcription.