Top left: Original repressilator with three genes consisting of an mRNA and a protein for each gene. Top center: Reduced notation of the repressilator with only the genes and the inhibitory connections shown. Top right: Histogram of the period with a fitted generalized Gamma distribution. Second row: Two different ways of interlocking two repressilators. The histogram of periods corresponds to the circuit in the middle column and has a higher mean but approximately the same coefficient of variation as above. Bottom row: Two symmetric circuits derived from the interlocking of four repressilators. The histogram of the periods for the circuit in the middle has a mean which is almost double that of the top histogram but the same coefficient of variation.