Housing supply was tight all over the country last year, and that’s a trend that certainly hasn’t changed much as we dive deeper into 2018. In fact, according to new data, the number of available homes for sale in January was the lowest we’ve seen since all the way back to 1999, and home price gains are continuing reflect the overall lack of inventory.

In 2017, U.S. home prices increased at the fastest 12-month pace since 2014 according to the latest S&P CoreLogic Case-Shiller national home price index, and it’s likely fewer people are listing their homes for sale because of the possibility of facing higher home prices, or even a higher mortgage rate.

The average 30-year mortgage rate most recently settled at 4.4%, which is still low from a historical standpoint. But also consider that rates have already jumped by 0.4% just since the beginning of 2018.

It’s worth noting, however, that much of the country also experienced bitter cold temperatures in January, which likely also played a role in January’s slowdown; so it’s too early to tell just how much increasing home prices and dwindling inventory are playing a role in current market conditions.

But if there are any two factors that can definitely slow down sales significantly, higher prices and higher mortgage rates are right there at the top of that list.