This is a term used to describe one possible type of backbone connection observed between beta strands in which the chain loops around to re-enter the sheet on the opposite end. For beta sheet connectivity, cross over connections are denoted by nx, where n denotes how many strands it moves over in the sheet and in which direction. +1x denotes a crossover connection between nearest neighbours, +2x denotes a crossover connection that misses one intervening strand in the sheet, and so on. The term 'crossover' has also been extended to include crossover loops which connect two sequential alpha helices that lie parallel in three dimensions.
The example shown is a transferase (CATH domain ID 1rwiA00)).
Proc Natl Acad Sci U S A73p2619-23(1976 Aug)
Nature268p495-500(1977 Aug 11)