Testing cancers for mismatch repair (MMR) gene defects assists selection of families for genetic testing for cancer predisposing Lynch syndrome. Performing MMR tumour testing without consent is debated, though little empirical work has been undertaken. We address this by examining the significance of testing for those who have consented without prior genetic counselling particularly their 'accounts' of testing via displays of knowledge and responsibility. Semi-structured interviews were conducted with participants recruited from a UK genetics service. Participants had difficulties in formulating the benefits of testing when consent was facilitated by a relative or by mail. Discourse analysis revealed that in the absence of specific or accurate understandings of tumour testing participants displayed responsibility towards themselves and others by generalising and diminishing the implications of testing. Within the framework of 'genetic responsibility', MMR tumour testing seems less important in participants' accounts compared to more definitive genetic testing. If facilitating genetic knowledge and responsibility are goals of genetic testing then developing appropriate consent procedures for MMR tumour testing is an important consideration. Further research is required to differentiate whether genetic counselling is a precondition for enacting genetic responsibility or whether intrinsic differences between types of testing make the concept less relevant.