Thanks to a large unbanked population and growing importance of the millennial customer, the digital-first bank business model is spreading not just in developed economies but also emerging markets like those in Asia.

And while this development is no big surprise to the financial services community, a new report from CB Insights titled “How 60+ Startups Are Disrupting Retail and Commercial Banking Around the World,” warns banks to pay close attention to these players.

“Traditional banking products, including checking, credit, and savings accounts, are under threat from a new crop of digital-first startups,” the report release last week states. And according to it, that the largest challenger banks around the world have a total of over 15 million users.

While that number might seem small, considering JPMorgan Chase alone has over 30 million mobile users, it is important for banks to take note. Perhaps through partnerships, these traditional banks can take advantage of the challenger bank ecosystem.

As stated in the report:

One of the biggest challenges these companies face is that many operate without a bank charter or license. As a result, they can only offer limited services.

This forces startups to enter into costly partnerships with incumbent financial institutions and creates dependency risk.

Another challenge is that many of these startups grow fast by offering free accounts and services to lower-income customers. While these customers may be easier to acquire, they are tougher to monetize.

But traditional banks are not oblivious to this competition. Larger banks across Europe like BBVA and Alior Bank have standalone mobile-first banks. In the U.S., there is JPMorgan’s Finn and Goldman Sachs consumer lending arm Marcus, and even credit unions like Florida-based SunCoast Credit Union, which previously told Bank Innovation that creating such a digital-first bank targeting millennial audience is on its roadmap.

And then there are developing markets across Asia and Africa that are goldmines for such challenger banks, according to the report.

“This cohort of 61 startups represents 14 countries across the globe. Africa and Asia are not yet well represented,” the report states. “Emerging markets represent some of the biggest opportunities for growth for digital banking because there are large populations of unbanked consumers with high rates of mobile adoption.”

Read the full report here.