The number of AChR fragments in NMJs of the mouse sternomastoid increases dramatically as the animals age. Cumulative histogram showing the percentage of fibers with the indicated numbers of separate fragments in the synaptic contact. The most substantial change occurs between 18 and 22+ months (a combination of ages 22–26 months), as the number of junctions with 10 or more fragments increases from ∼25 to 80%. Number of animals/number of fibers are as follows: P21, 2/121; P42, 2/94; P70, 2/62; P140, 1/51; postnatal 11 months, 2/68; postnatal 18 months, 1/52; postnatal 22–26 months, 4/141.