  We investigate the dynamics of the heterodimer autorepression loop (HAL), a
small genetic module in which a protein A acts as an auto-repressor and binds
to a second protein B to form a AB dimer. For suitable values of the rate
constants the HAL produces pulses of A alternating with pulses of B. By means
of analytical and numerical calculations, we show that the duration of A-pulses
is extremely robust against variation of the rate constants while the duration
of the B-pulses can be flexibly adjusted. The HAL is thus a minimal genetic
module generating robust pulses with tunable duration an interesting property
for cellular signalling.
