Immunohistochemical expression of Hsp27, Hsp72, Hsp73 and Hsp90 was evaluated in 3 normal canine mammary glands and 30 malignant mammary tumours (10 in situ carcinomas, 10 invasive carcinomas limited to local structures without identifiable invasion of blood or lymphatic vessels, 10 carcinomas with invasion of blood or lymphatic vessels and/or metastases to regional lymph nodes). The gene discussed is HSP90AA1; the disease is invasive carcinoma.