  A graph $G=(V,E)$ with a vertex set $V$ and an edge set $E$ is called a
pairwise compatibility graph (PCG, for short) if there are a tree $T$ whose
leaf set is $V$, a non-negative edge weight $w$ in $T$, and two non-negative
reals $d_{\min}\leq d_{\max}$ such that $G$ has an edge $uv\in E$ if and only
if the distance between $u$ and $v$ in the weighted tree $(T,w)$ is in the
interval $[d_{\min}, d_{\max}]$. PCG is a new graph class motivated from
bioinformatics. In this paper, we give some necessary and sufficient conditions
for PCG based on cut-vertices and twins, which provide reductions among PCGs.
Our results imply that complete $k$-partite graph, cactus, and some other graph
classes are subsets of PCG.
