Jim Kelly, Steve Young, Reggie White, and Herschel Walker all cut their professional teeth in it.

And without it, the NFL wouldn't have instant replay, two-point conversions or the Jacksonville Jaguars.

It's hard to believe that it's been 20 years since the USFL folded, but its legacy lives on in its former players, the rule changes it inspired, the markets it introduced, and the card sets it spawned.

So popular is the 1984 Topps USFL set, in fact, that some hobbyists have called it the most coveted gridiron issue of the '80s.

"With its list of rookie cards and the sheer unique nature of the USFL, it stands out as the top set of the '80s as far as I'm concerned," said Steven Daniels, a collector based in Kennett, Mo., whose 1984 USFL set ranks fourth on the PSA Set Registry.

Produced solely in factory sets, the 132-card issue offers the rookie cards of the aforementioned Kelly, Young, White, and Walker. Also included are the inaugural singles of Ricky Sanders, Vaughan Johnson, Anthony Carter, and Bobby Hebert.

Reportedly printed in significantly smaller quantities than its 1984 NFL counterpart (though Topps spokesperson Clay Luraschi could not confirm this), the USFL product has been a challenge for collectors to finish in high grade. So far, no one has listed a complete set on the PSA Set Registry.

Some hobbyists have compared the set's production numbers to those of the storied 1984 Fleer Update Baseball set, but Daniels says that in his experience, cards from the USFL issue are more difficult to obtain.

"I definitely see more Fleer baseball. I think you just had a broader base of baseball card collectors at that time," he said.

On top of its scarcity, this set is also hampered by a number of condition issues. One of the most common flaws is poor centering.

"I've just noticed centering issues generally in the whole set," said John Vineyard, a California-based enthusiast whose set is more than 65 % complete and sits at No. 1 on the registry.

The Young rookie is particularly difficult to find centered, confirms Roger Rumsey, co-owner of 4 Sharp Corners in Foxboro, Massachusetts. Of the 862 Young cards submitted to PSA, only around 9% of them have garnered a PSA 9 Mint grade. In contrast, close to 26% of Kelly rookies have earned a PSA 9.

"I think a lot of it has to do with how they got cut and where they were on the run (on the sheet)," said John Brewster Ohle, III, a collector based in Chicago whose set is No. 2 on the registry.

Topps spokesperson Clay Luraschi says that the location of the Young card on the sheet may, indeed, be the reason for its centering problems, but he couldn't definitively confirm this.

Another common hindrance with these cards is damaged corners. Hobbyists attribute this to the flimsy boxes that the product was sold in.

"I think there are some corner issues with the set in general... Those boxes tend to get banged up over the years," said Vineyard.

Daniels agrees.

"I am not sure about the distribution methods that Topps employed with these sets, but it just seems from that, many times, the box that houses this set didn't make it through the distribution process very well. I have encountered several sets with the box having a pinched edge, resulting in dinged corners or various sorts of edge damage."

Hobbyists say that the trademark pink backs of these cards are also vulnerable to damage.

"I've noticed a lot of flaking on them (the pink backs)... A lot of the cards that I have received back from PSA in lower grades have been, as far as I could tell, for flaking, because they'll be just gorgeous otherwise," said Vineyard.

Collectors should also be wary of Jim Kelly counterfeits. Authentication experts at PSA say that you should examine the trademarks (TM) on the card's front (one "TM" is located at the top to the right of the word "USFL"; the other is located in the bottom left below the team helmet) and the card number on the back. The "TMs" and card number should be solid and bold on authentic examples. If they appear grainy, the card may be a counterfeit.

Aside from the Young, there are a number of cards that are difficult to uncover in high grade. In fact, there are an abundance of cards that have yet to be submitted to PSA for grading. One of these is Luther Bradley (Card #1), a single that is likely to be condition sensitive because it's the first card in the set.

Unlike the Bradley, however, the final card - a checklist (Card #132) - has been submitted eight times. Vineyard says that the only PSA 9 Mint checklist recently sold for $250 on eBay.

"That's the last card in the set. In the boxed set, it's always right up there against the edge and it's getting beat up..." he said.

With Kelly and Young recently inducted into the Hall of Fame, and White a sure bet to be enshrined, this set has grown in popularity in recent years.

"I have noticed an increase in the interest level for this set," said Daniels. "I think it was inevitable as more of the key players are nearing Hall of Fame status..."

On top of boasting Hall of Famers, this set is also coveted for nostalgic reasons.

Brewster Ohle, III is from New Orleans and he remembers taking photos of the USFL's New Orleans Breakers for the local paper as part of a high school assignment.

"It was kind of something new, something different, something exciting," he recalled of the ill-fated circuit.

And for many hobbyists, the 1984 Topps USFL set continues to be just that - "something different" and "something exciting."