It requires a long-term mindset, but buy-and-hold investing is the best way to build wealth. It reduces costly transaction fees, allows the greatest tax benefits, and reduces the temptation of mixing emotions with money. Even better, the process of buying great businesses at great prices and holding on for the long haul goes even further when growth stocks are involved.

Where should you start? Well, how about two businesses with excellent track records in a high-growth sector? Repligen (NASDAQ:RGEN) has delivered impressive growth by developing, acquiring, and selling bioprocessing products that help biopharma companies research, study, and commercialize biologic drugs. Catalent (NYSE:CTLT) manufactures drug products for customers across the pharmaceutical and biopharmaceutical sectors and recently made a huge investment in gene therapy manufacturing. 

Here's why these are two growth stocks to buy and hold for decades. 

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Leave the risks of biopharma behind

Repligen offers a diverse portfolio of products used to grow cell cultures, measure bioprocess metrics during manufacturing, and purify products downstream. Investors don't have to be technically competent to appreciate the role that the company plays in the high-growth biopharma sector. After all, if biotech companies cannot safely and efficiently manufacture drug products, then they won't be in business very long. 

The bioprocessing leader is relatively small compared with the better-known players in the space, such as General Electric's GE Healthcare subsidiary, Samsung BioLogics, Thermo Fisher Scientific, and Sartorius. But Repligen has done a great job making the most of its high-margin niche and geographic proximity to Boston, a world-leading biotech hub.

In the first nine months of 2019, Repligen reported revenue of $200 million and operating income of $30.2 million, for year-over-year growth of 41% and 66%, respectively. The business exited September fresh off its largest-ever acquisition and, after stock offerings, still had a record cash balance of $513 million. Shares gained 75% in 2019, easily topping the roughly 29% gain of the S&P 500

Investors should have little doubt that the business can maintain its momentum. In 2018, the Food and Drug Administration approved a record 11 monoclonal antibodies (the main category of biologic drug), while more than 400 more were in development across the industry's pipeline. Combine that with gene therapies, other genetic medicines, and cellular medicines that receive billions of dollars in investment across the biopharma sector, and Repligen has "long-term investment" written all over it. 

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A huge bet on gene therapy should pay off

The FDA expects to receive more than 200 investigational new drug (IND) applications for gene therapies in 2020. That signals the growing importance of the therapeutic approach. It also suggests that Catalent has a formidable growth opportunity ahead of it. 

Last year, Catalent acquired Paragon Bioservices for $1.2 billion. The business, now accounted for in the biologics segment, was expected to notch over $200 million in revenue for the calendar year 2019 (Catalent's fiscal year ends on June 30). Paragon is a leading manufacturer of viral vectors (which can also be used to deliver certain CRISPR-based drug payloads) and other gene therapy products. If genetic medicines live up to their lofty potential, then the $1.2 billion acquisition price could look like a steal years from now.

In the fiscal first quarter of 2020, which was the first full period that Paragon contributed, the biologics segment reported $188.6 million in revenue. That was 28% of total revenue for Catalent and marked 51% growth from the year-ago period. 

Despite the growth opportunity in gene therapy, investors will appreciate the company's diverse revenue base. Catalent's bread and butter remains softgel delivery capsules, which the company pioneered in the 1930s and has improved ever since. The segment was responsible for nearly 40% of total revenue in the most recent quarter and over half of total EBITDA. It might be boring and easy to overlook, but the segment notched year-over-year revenue growth of 10% in the fiscal first quarter of 2020. The company leans on its oral and specialty delivery segment and clinical services segment for the remaining 33% of revenue. 

Long-term investors should find plenty to like about Catalent. It made a big, bold bet in gene therapy manufacturing that could pay off handsomely in the next decade and position the biologics segment as the company's largest. If and when that occurs, the business will still generate a healthy amount of revenue and earnings from diverse, unrelated product portfolios. Simply put, the growth stock is a low-risk way to own a piece of the pharma and biopharma sectors.