  We analyze the current-biased Shapiro experiment in a Josephson junction
formed by two one-dimensional nanowires featuring Majorana fermions. Ideally,
these junctions are predicted to have an unconventional $4\pi$-periodic
Josephson effect and thus only Shapiro steps at even multiples of the driving
frequency. Taking additionally into account overlap between the Majorana
fermions, due to the finite length of the wire, renders the Josephson junction
conventional for any dc-experiments. We show that probing the current-phase
relation in a current biased setup dynamically decouples the Majorana fermions.
We find that besides the even integer Shapiro steps there are additional steps
at odd and fractional values. However, different from the voltage biased case,
the even steps dominate for a wide range of parameters even in the case of
multiple modes thus giving a clear experimental signature of the presence of
Majorana fermions.
