In vivo LNP-mRNA encoding CARs or multiplexed T-cell engagers (e.g., Moderna’s mRNA-2808 encoding BCMA/FcRH5/GPRC5D TCEs, now in Phase 1/2) bypass ex vivo manufacturing entirely, allowing rapid redosing/switching of targets with ultra-low toxicity and no genomic risk—ideal for monitoring/adapting to clonal evolution and antigen density shifts during progression (MGUS → SMM → MM → relapse; Figure 3). Here, FCRL5 is linked to Miyoshi myopathy.