UNC-GRK4-V is expressed in AML. (A) GRK4 has limited tissue expression, with transcripts only detectable by RT-PCR in human AML and testis, but not in PBMCs, liver, colon, or skin. (B) Western blot confirms GRK4 protein expression in testis and 3 of 4 human AML samples. (C) Extracted ion chromograms using DIMS identifies the E_C to allow the maximum signal from UNC-GRK4-V (m/z = 1049.5) into the MS using the pure standard (red trace), which was 86 V/cm. Extracted ion chromogram of m/z = 1049.5 in the U937.A2 cell epitope pool (blue trace) shows that other species with m/z = 1049.5 can be detected across a range of E_C. (D) The fragmentation pattern for pure UNC-GRK4-V peptide was determined for comparison with results from the peptide pool. (E) Targeted MS was performed on the epitope pool by setting E_C = 86 V/cm and fragmenting the parent ion with an m/z = 1049.5. The resulting MS/MS spectrum is virtually identical to that of the pure peptide.