Abstract: A mullion connection system for resisting wind load reactions using a connection clip structurally engaged with a mullion without a hole or a fastener penetrating into the mullion surface. The connection clip can undergo stress-free vertical sliding relative to the mullion along the entire length of the mullion without impairing the structural connection strength; therefore, the building construction tolerance in the vertical direction is eliminated as a design consideration of the curtain wall system. The design also eliminates the need for pre-fabricating mullion holes for the connection locations. The design does not require the pre-determination of the elevation of the connecting point and thus is most beneficial for a curtain wall renovation project where the available locations in the building structure for structural connection cannot be determined prior to the demolition of the existing wall.