  Spontaneous CPT breaking arising in string theory has been suggested as a
possible observable experimental signature in neutral-meson systems. We provide
a theoretical framework for the treatment of low-energy effects of spontaneous
CPT violation and the attendant partial Lorentz breaking. The analysis is
within the context of conventional relativistic quantum mechanics and quantum
field theory in four dimensions. We use the framework to develop a
CPT-violating extension to the minimal standard model that could serve as a
basis for establishing quantitative CPT bounds.
