The present disclosure relates to a fiber optic telecommunications cable assembly including a main fiber optic cable and a tether cable that branches from the main fiber optic cable at a breakout location. The fiber optic telecommunications cable assembly also includes a breakout block mounted to the main fiber optic cable at the breakout location, and an over-mold that covers the breakout block and at least a portion of the main fiber optic cable. The breakout block defines a straight-through channel in which the main fiber optic cable is received and a breakout channel that branches out from the straight-through channel. The breakout block includes seams with overlap configurations that prevent the over-mold from entering the breakout block through the seams. The breakout block also includes barrier dams for preventing bonding material from entering the breakout channel.