In 2007, 2010 and 2011, the European Food Safety Authority (EFSA) Panel on Genetically Modified Organisms (GMO Panel) concluded the assessment of genetically modified (GM) maize GA21, MIR604 x GA21 and MIR604 x GA21 x Bt11. These were found to be as safe as their conventional counterparts and other appropriate comparators with respect to potential effects on human and animal health and the environment. On 23 July 2015, the European Commission (EC) received from Syngenta new nucleic acid sequencing data on maize event GA21 and updated bioinformatic analyses using the new sequencing data. The EC tasked EFSA to analyse these data and to indicate whether the previous conclusions of the EFSA GMO Panel on the above-listed GM maizes remain valid. The EFSA GMO Panel used the appropriate principles described in its guidelines for the risk assessment of GM plants to analyse the received data. Compared with the sequencing data originally provided, the new sequencing data indicated a one-base pair addition in a non-coding region of the insert, a three-base pair deletion in the 3’ flanking region of the insert, and a difference in the number of functional copies of the mepsps expression cassette. These differences were only recently identified, but it was confirmed that they had been present in the original plant material used for the risk assessment. Thus, with the exception of bioinformatics analyses, the studies performed for the risk assessment remain valid. The bioinformatic analyses performed on the new sequence did not give rise to safety issues. Therefore, the GMO Panel concludes that the original risk assessment of event GA21 as a single event, and as a part of stacked events, remains valid.