KRAS and neoplasm: To assess circulating KRAS (ctKRAS) as a surrogate biomarker for tumor burden in FAS patients, KRAS mutant allele frequency (MAF) was determined in ctDNA obtained from blood samples (n = 46) and compared with the sum of the longest diameter (SLD) of tumor lesions over the first 300 days of treatment (Figure 5A; Figure S6).