RAB27A and neoplasm: Interestingly, inhibition of the small GTPase Rab27a in 4T1 tumor cells decreased CD63 levels in all fractions (i.e., to less than the detection limit), whereas levels of CD9 and Mfge8 were only moderately reduced Immuno electron microscopy allowed the authors to study the distribution of CD9 within the seemingly heterogeneous EXO populations in more detail.