NTRK1 and Merkel cell skin cancer: Furthermore, Western blots also detected a 100-kDa TrkA isoform, consistent with Δ exon 6–7 TrkAIII protein, in protein extracts from the metastatic MCC, which was not detected in a normal skin extract and was of an identical molecular size to the 100-kDa Δ exon 6–7 TrkAIII expressed by stable Δ exon 6–7 TrkAIII-transfected SH-SY5Y cells compared to 140-kDa fully spliced TrkA expressed by stable TrkA-transfected SH-SY5Y cells pcDNA-transfected SH-SY5Y cells (negative control) [29].