IT HAS BEEN seven years and something near $1 billion invested. Now, finally, the first products of gene engineering have begun to arrive on the market.

As it turns out, though, the products are a good deal more mundane and practical money-makers than the news-prose over the past few years may have led many to expect.

There are a number of medically important substances among the first products of the gene machinery, such as interferon and blood-clotting factors. But genes begin to sound more like business and less like medical science as products such as a vaccine for cow warts and a hormone to increase bovine milk production appear to outnumber the medical miracles going to market.

Take insulin. It is the first product made by splicing genes -- human and bacterial in this case -- to be marketed for use in humans. Genentech put the genes together, and Eli Lilly put together the manufacturing and the advertising campaigns aimed at doctors.

Until now, insulin has come from pig or cow pancreas. The animal varieties of insulin are chemically only a molecular jot away from the human variety, and in clinical trials there was no significant difference found between the two.

But the motive for making "human" insulin is not mostly medical. (It's not, for that matter, strictly speaking, human. Bacteria make it.) It is in the advertising possibilities. If you were diabetic and could buy pig insulin or "human" insulin at about the same price, which would you choose? It is an advertising man's dream.

The chief insulin market Eli Lilly is going after with this first product is Britain, where Novo Industry is the dominant company in the insulin field. (Lilly already has 85 percent of the U.S. market with its own animal insulin.)

Novo has not taken the bacterial "human" insulin lying down. It has worked a chemical trick that, without gene engineering, renders animal insulin "humanized." "Humanized" insulin has just begun to do battle in Britain, hitting the market a few months quicker. But "human" is coming in 10 percent cheaper.

Among the other gene-engineered products that will be ready for the market within the next year or so will be such items as pig-, cow-, and sheep-growth hormones. The idea here is not to make gigantic farm animals, but chiefly to get a little more growth per pound of feed. In the case of cows, growth hormone also seems to increase milk production. In sheep it increases wool growth.

One product a Midwestern gene company is planning to bring out is a vaccine to prevent warts in cattle.

"Warts are not a huge problem," a company spokesman says. "It's a small specialty market -- less than a million doses a year." But for animals going to shows, something must be done about warts. And there are a few serious cases of wart infection, such as those which cover the udder and make milking difficult.

Now, the only sort of vaccine available is one made up by grinding up whole warts and injecting the result to get the immune system's defenses up. By gene engineering, a more sophisticated vaccine is possible.

There are dozens more products that may be ready for the market within a year or two. They include:

* Cow interferon. Interferon's chief power is as a virus-stopping agent. A viral disease among cattle called "shipping fever" causes illness in a large percentage of shipped cattle, bringing on weight loss or worse, and millions of dollars of losses. Cow interferon may help.

* Human growth hormone. May prevent dwarfism, but also may quicken the healing of fractures or aid in burn therapy.

* Serum albumin. Albumin is a component of blood needed to treat patients going through surgery or those with severe blood loss.

* Animal diarrhea vaccines. Both cattle and pigs can suffer severe diarrhea as newborns. More than 5 percent die. The market for an effective vaccine may be $30 million annually.

* Engineered bacteria. In industries using bacteria to convert starch into alcohol, a bacteria with multiple, enhanced genes to carry out the process can make production more efficient. More yield of alcohol on less bacterial food.

* Aspartic acid. It is a component of aspartame, the newly approved artificial sweetener that is expected to be a serious competitor to saccarin.

* Hepatitis diagnosing kit. A quicker, more effective method of diagnosing hepatitis.

These first products are virtually all copies of products already on the market, but have the advantage of being cheaper and easier to make in quantity. Future products promise more of the same, though eventually there will be one or two that are really new, such as the antibody molecules that will be able to find and destroy virtually any molecular target in the body, in a polluted waterway, or in a chemical company's vat.

But we will have to wait a while for these once-impossible products to emerge.