After observing that the viability of HT1080 fibrosarcoma cells was suppressed by ASC when in close proximity to neighboring cells, we examined several signaling pathways to reveal that ASC suppressed XIAP expression, enhanced the involvement of gap junctions, and consequently activated caspase-9, which resulted in the induction of apoptosis, and not necrosis, in HT1080 fibrosarcoma cells via close interactions with adjacent cells in high density conditions. Here, CASP9 is linked to fibrosarcoma.