  A fast transition between a standard matter-like era and a late
$\Lambda$CDM-like epoch generated by a single Unified Dark Matter component can
explain the observed acceleration of the Universe. UDM models with a fast
transition should be clearly distinguishable from $\Lambda$CDM (and
alternatives) through observations. Here we focus on a particularly simple
model and analyse its viability by studying features of the background model
and properties of the adiabatic UDM perturbations.
