Scottish Mortgage Investment Trust (LSE: SMT) shares make up a significant part of my ISA. Thankfully, they’ve been rising in recent months and now trade for £9. But that’s some way off the £15 mark they reached back in 2021.

Still, if I had £2k gathering dust in my ISA right now, I’d snap up this FTSE 100 stock to hold for the long term. Here’s why.

Finding big winners

Scottish Mortgage is a growth-focused investment trust with 98 holdings. Many of these are household names like Tesla, Amazon, Netflix, and Spotify.

The trust also invests in private companies, which currently account for around 27% of assets. Some well-known ones here include Elon Musk’s SpaceX, TikTok parent company ByteDance, and Epic Games, the maker of Fortnite.

The biggest investing theme over the last two and a half years has been artificial intelligence (AI), of course. Nvidia has become the posterchild for the AI revolution, posting truly eye-popping increases in both revenue and profits.

The stock is up an incredible 3,480% in just the past five years!

Scottish Mortgage first invested in the AI chipmaker in July 2016 when its shares stood at a split-adjusted $1.42. That means the trust’s holding has risen 90 times in value since the initial purchase!

Any growth-oriented fund worth its salt ought to have owned Nvidia stock over at least the last couple of years. So it’s reassuring that the trust was way ahead of the curve on this one.

Identifying the next Nvidia

However, it’s also reassuring for shareholders that Scottish Mortgage is not resting on its laurels. Already, the managers are busy trying to identify the next crop of big winners.

Deputy manager Lawrence Burns recently highlighted three early-stage companies held in the company that appear to have massive long-term potential.

• Bill Gates-backed Upside Foods, which grows meat from animal cells
• Climeworks, which removes carbon dioxide from the air using advanced carbon-capture technology
• PsiQuantum, a $3.1bn quantum computing start-up

Now, it goes without saying that there’s no guarantee such private holdings will ever become the next big winners. Indeed, they carry extra risk as young companies and can be harder to accurately value.

Meanwhile, if anything goes wrong at some of the bigger private holdings — a rocket disaster at SpaceX, for example, or TikTok being banned in the US — that might negatively impact the trust’s value.

Investing at a discount

Nevertheless, Scottish Mortgage looks like great value to me today. It’s trading at a 9% discount to its net asset value (NAV). So, in theory, I can get access to the high-growth companies held by the trust at a cheaper value than their current market price.

In March, the trust’s board sanctioned a whopping £1bn share buyback programme to run over the next two years. This is the largest buyback by any investment trust and is aimed at narrowing the NAV discount.

Plus, as the stock chart shows, Scottish Mortgage shares are still 41% off their all-time high, despite the recent revival. That looks attractive to me.

Therefore, if I had a spare couple of grand lying around, I’d get in now while the stock is trading at a significant discount. It looks like a FTSE 100 bargain to me.