NTRK1 and Merkel cell skin cancer: Indirect IF analysis of MCPyV-positive FFPE MCC tissues also detected IF immunoreactivity for intracellular TrkA protein isoforms and variable immunoreactivity for tyrosine-phosphorylated TrkA isoforms, consistent with potential intracellular Δ exon 6–7 TrkAIII protein expression and activation (Table 3).