The MX-5's first generation, the NA, sold over 400,000 units from May 1989 to 1997—with a inline-four engine to 1993, a engine thereafter (with a de-tuned 1.6 as a budget option in some markets)—recognizable by its pop-up headlights. The second generation (NB) was introduced in 1999 with a slight increase in engine power; it can be recognized by the fixed headlights and the glass rear window, although first generation owners may opt for the glass window design when replacing the original top. The third generation (NC) was introduced in 2006 with a engine and slightly larger body size, with a Power Retractable Hard Top variant added in 2007. The fourth generation (ND) was introduced in 2015 with a new SKYACTIV-G engine and a body size similar to the first generation model, with a Retractable Fastback (RF) variant added later that year.