Thanks @erictgrubaugh. I will have a checkbox in the first column (for users to select a whole row to export to CSV) and then 2 other columns that are checkboxes but represent other things ('Is Sample Ok?', 'Is TRF Form ok?'). When I click 'Mark all' it ticks all 'Is Sample Ok?' and 'Is TRF Form ok?' to on ('T'). Is that meant to happen? I only want the first checkbox column to get ticked on.