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The appearance of lesional cells is in between fibroblasts and primitive mesenchymal cells. Some areas are cellular with brisk mitotic activity and have to be distinguished from infantile fibrosarcoma. In infantile fibromatosis, highly cellular areas alternate with more hyalilinized foci. It also lacks the cytogenetic alteration found in infantile fibrosarcoma: t(12;15)(p13;q25) resulting in the fusion of ETV gene with NTRK3 gene.