It's official. The human body uses a Mark and Sweep algorithm for garbage collection. From the Nobel Prize for Chemistry Press Release:

Aaron Ciechanover, Avram Hershko and Irwin Rose have brought us to
realise that the cell functions as a highly-efficient checking
station where proteins are built up and broken down at a furious
rate. The degradation is not indiscriminate but takes place
through a process that is controlled in detail so that the
proteins to be broken down at any given moment are given a
molecular label, a 'kiss of death', to be dramatic. The labelled
proteins are then fed into the cells' "waste disposers", the so
called proteasomes, where they are chopped into small pieces and
destroyed.

The label consists of a molecule called ubiquitin. This fastens to
the protein to be destroyed, accompanies it to the proteasome
where it is recognised as the key in a lock, and signals that a
protein is on the way for disassembly. Shortly before the protein
is squeezed into the proteasome, its ubiquitin label is
disconnected for re-use.