  We elaborate on an extension of the Standard Model with a gauge structure
enlarged by a single anomalous U(1), where the presence of a Wess-Zumino term
is motivated by the Green-Schwarz mechanism of string theory. The additional
gauge interaction is anomalous and requires an axion for anomaly cancelation.
The pseudoscalar implements the St\"{u}ckelberg mechanism and undergoes mixing
with the standard Higgs sector to render the additional U(1) massive. We
consider a 2-Higgs doublet model. We show that the anomalous effective vertices
involving neutral currents are potentially observable. We clarify their role in
the case of simple processes such as $Z^*\to \gamma \gamma$, which are at
variance with respect to the Standard Model. A brief discussion of the
implications of these studies for the LHC is included.
