BRD2 and cancer: Although CA19-9 level change is an effective surrogate marker of response, as many as 18% to 34% of patients with PDAC have Lewis α-β- (negative) blood genotype and present with a normal level of CA 19-9 (<37 U/ml).46 In the absence of a dependable biomarker, histopathologic grading of the extent of residual cancer may be used as a surrogate marker of response to NAT.29,30,31,34,47,48 Chatterjee et al34 concluded that the grade and extent of residual tumor is critical in predicting prognosis in PDAC treated with NAT.