Conversation so far:
Who were Dion and the Belmonts?
Dion and the Belmonts were a leading American vocal group of the late 1950s.
Who were Dion and the Belmonts' members?
The established trio of Angelo D'Aleo, Carlo Mastrangelo, and Fred Milano, formed a quartet with Dion DiMucci.

Question: What happened in 1957?
Self-contained rewrite: What happened in 1957 to Dion and the Belmonts?