Massive degeneration of granule cell precursors occurs perinatally in the cerebellum of weaver mutant mice. We have studied the electron microscopic (EM) features of granule cell death in weaver and control mice, using an in situ end-labeling (ISEL) technique for detecting DNA fragmentation, a hallmark of apoptosis. In all animals, EM-ISEL revealed the pattern of apoptosis, with an enhanced expression in weaver mice. The weaver gene appears to accelerate the death program, most likely through potassium channel-mediated signals.