Apparatus for comparing an input genome sequence with one or more reference sequences comprises signal generators (10) for generating an input signal for each occurrence of each base component type (A,C,G,T) of the input sequence. A respective optical correlator (23-26) is provided for each base component type. Each correlator detects a match or near match between the occurrences of the respective base component type in the input sequence and the occurrences of that component type in any of the reference sequences, and produces a respective correlation signal. Decision-making circuitry (31) responds to the correlation signals from all of the correlators to provide an indication of a match or near match between the whole input sequence and any of the reference sequences.