In degus (mean age: 4.2 years old, age range: 1.5–6 years; females: four; males: three; sex unspecified: one), except for one case of epitheliotropic T-cell lymphoma (CD3+), all tumours were soft tissue sarcomas: fibrosarcoma—two cases; poorly differentiated sarcoma (vimentin, clone V9+, Iba-1-, desmin-, α-SMA-)—two cases; myxosarcoma—one case; anaplastic sarcoma with giant cells (vimentin, clone V9+, Iba-1-, desmin-, α-SMA-; Figure 5)—one case; and malignant peripheral nerve sheath tumour—one case. The gene discussed is ACTA1; the disease is fibrosarcoma.