Men aged ≥18 years, women aged ≥55 years, and women <55years who are sterilised, or have had a hysterectomy or have effective contraception and thus where there is no possibility of them being pregnant; presenting within 12 hours of the onset of chest pain who have ST segment elevation of more than 1mm elevation in limb leads or 2mm elevation in two contiguous chest leads or new left bundle branch block (LBBB) and for whom the clinical decision has been made to treat with primary PCI will be eligible for enrolment. Occlusion of the culprit related artery (TIMI grade 0 or TIMI grade 1) will also be required for inclusion. Eligible patients will be of North European descent.