Function of spi-1l in myelopoiesis. Proposed model of spi-1l function during myelopoiesis. Spi-1l functions downstream of cloche, etsrp, and pu.1 and is regulated by these transcription factors. Spi-1l is necessary and sufficient for mpx and lcp1 expression. At this time, we cannot differentiate whether spi-1l activates mpx and lcp1 expression together or sequentially with spi-1.