We present a brief review of molecular biological basis and mathematical modelling of circadian rhythms in Drosophila. We discuss pertinent aspects of a new model
that incorporates the transcriptional feedback loops revealed so far in the network of the
circadian clock (PER/TIM and VRI/PDP1 loops). Conventional Hill functions are not used
to describe the regulation of genes, instead the explicit reactions of binding and unbinding
processes of transcription factors to promoters are probabilistically modelled. The model
is described by a set of ordinary differential equations, and the parameters are estimated
from the in vitro experimental data of the clocks' components. The model is robust over a
wide range of parameter variations. Through the sensitivity analysis of the model, roles of
VRI and PDP1 feedback loops are investigated, and it is proposed that they increase the
robustness of the clock.