I've said it so many times, I'm starting to feel hoarse. Or like I'm beating a dead horse.

For years, I've argued that stem-cell concern ViaCell (NASDAQ:VIAC) is actually two companies. One is a profitable, tollbooth-like business that collects samples of umbilical-cord blood and charges a recurring fee for their storage. The other is an unprofitable, cash-burning disaster of a biotech arm that researches pie-in-the-sky therapies using stem cells. If there's any doubt remaining, I like the first business; I hate the second.

Considering last week's Q4 and full-year earnings results, I wouldn't be surprised to learn that other investors feel the same. On the plus side -- and not coincidentally, the tollbooth side -- ViaCell's investments in sales and marketing do, in fact, appear to be paying off in the form of rapidly growing sales. Clocking in at $14.4 million for the quarter, sales of the ViaCord storage business grew 30% in comparison with Q4 2005's numbers.

Growth is good
Sound good? It gets better. ViaCord sales were up 28% in the second half (meaning growth accelerated as the year progressed), and 24% for the year as a whole. Incidentally, these growth trends stand in stark contrast to those of publicly traded archrival Cryo-Cell (OTC BB: CCEL), which, as we discussed last week, saw its own growth hit the brakes in Q4.

Slower growth (in some things) is also good
Better still, ViaCell is beginning to enjoy the benefits of scale in its ViaCord business. In the fourth quarter, the company announced that the cost of processing and storing its growing trove of stem cells increased 19% in comparison with last year's Q4 -- slower than sales grew. Better still, the cost increased 24% for the year as a whole -- keeping pace with sales growth for the year and keeping margins stagnant. The fact that ViaCord's sales began to pull away from its costs in Q4 tells us to expect continued margin expansion as time progresses.

Also attractive is that selling, general, and administrative expenses, while still growing uncomfortably faster in Q4 than the sales they're intended to inspire and support, decelerated from the full year's pace, dropping from 44% growth to 32% growth in Q4.

But it can be bad, too
The bad news? What looked like a trend in 2006 may not play out in 2007, if management's guidance is to be believed. According to ViaCell CEO Marc Beer, in the first half of 2007, we're to expect growth "consistent with its growth in 2006 compared to 2005" -- i.e., the "slower" half of 2006. Granted, the firm expects a new marketing initiative to repeat last year's second-half growth spurt as well.

For my part, I'd much prefer to see growth not take a step back in the interim. Unfortunately, that's exactly what ViaCell is promising.

Further reading material available in the science lab:

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Fool contributor Rich Smith does not own shares of any company named above.