121 (MBTA bus)

The Massachusetts Bay Transportation Authority (MBTA) operates local buses in East Boston, Boston, Massachusetts, connecting to the Blue Line at Maverick or Wood Island. Additionally, the Silver Line SL1 bus and 171 (early morning only) serve Logan Airport, and the 424, 434, 441, 442, 448, 449, 450, 455 and 459 express buses pass through on Route 1A on their way between North Shore points and downtown.

Some of these routes run through Chelsea. The 111 also serves Chelsea without entering East Boston. The 426 and 428 express buses pass through Chelsea on the Northeast Expressway.

The 119 runs in neither city, only running in Revere, but is considered part of the same group.

...
Wikipedia