A plug-in part for a plug connector arrangement is provided. The plug-in part being in the form of a piece of tubing and having a sealing portion, comprising a sealing element enclosing the piece of tubing in a circumferential direction, and a locking portion with at least one undercut, and it being possible for the plug-in part to be plugged into the receptacle of a coupling part of the plug connector arrangement in order to produce a fluid-tight connection, and for at least one locking element of the coupling part to engage behind the plug-in part in the region of the undercut. In order to develop the plug-in part so as to reduce the risk of the sealing element being adversely affected, the undercut is formed by a protrusion which projects from the outside of the piece of tubing and extends circumferentially merely over a sub-region of the piece of tubing.