PTK6 and breast carcinoma: The cytoplasmic non-receptor tyrosine kinase (BRK, PTK6), originally cloned from a human metastatic breast tumour (Mitchell et al, 1994), shows elevated expression in breast carcinoma cell lines and in approximately two-thirds of primary breast tumours (Mitchell et al, 1994; Barker et al, 1997; Llor et al, 1999; Born et al, 2005).