CD99 and neoplasm: CD99 engagement induces death in tumor cells via non-conventional, caspase-independent programmed cell death or through a non-apoptotic pathway resembling methuosis (Cerisano et al. 2004; Manara et al. 2016; Scotlandi et al. 2000; Sohn et al. 1998), a process characterized by excessive accumulation of vacuoles in the cytoplasm, leading to compromised cell viability (Maltese and Overmeyer 2014).