Binding of the RUNX1:CBFB complex to the promoter of the PF4 gene stimulates transcription of PF4. The PF4 gene encodes Platelet factor 4, a protein stored in platelet alpha granules. Deficiency of alpha granule proteins, including PF4, is the cause of gray platelet syndrome. PF4 deficiency can be caused by RUNX1 haploinsuficiency (Aneja et al. 2011).