Table:
|  |  | Three Months Ended |  | Variation |  |
| --- | --- | --- | --- | --- | --- |
|  | December 31, 2019 | September 29, 2018 | December 31, 2018 | Sequential | Year-Over-Year |
|  |  | (Unaudited, in millions) |  |  |  |
| Gross profit | $ 1,081 | $ 967 | $ 1,059 | 11.8% | 2.0% |
| Gross margin (as percentage of net revenues) | 39.3% | 37.9% | 40.0% | +140 bps | -70 bps |
Paragraphs:
Fourth quarter gross profit was $1,081 million and gross margin was 39.3%. On a sequential basis, gross margin increased 140 basis points, mainly driven by improved product mix and better manufacturing efficiencies.
Gross margin decreased 70 basis points year-over-year, mainly impacted by price pressure and unsaturation charges, partially offset by improved manufacturing efficiencies, better product mix and favorable currency effects, net of hedging.
Question:
What led to the increase in the gross margin on a sequential basis?
Answer: mainly driven by improved product mix and better manufacturing efficiencies.