KRAS and colorectal carcinoma: Boos et al. exposed KRAS and BRAF wild-type PDOs from three patients (one primary CRC metastatic tumor and two CRC liver metastases) to a chronic combination treatment of FOLFIRI + cetuximab, administered concomitantly, to establish ex vivo chemoresistance within a variable patient-dependent period from 4 to 6 months [48].