A transistor array used in exponential converter circuits in a number of analog synths, including the most common version of the Minimoog. It was also used in, among other things, early Buchla VCOs, early Roland synth models including the Jupiter 4, and most EML synths. What made the uA726 valuable for expo converter circuits was that it contained an onboard heater and thermostat mechanism to keep the chip temperature stable, eliminating the inherent temperature variability that is present in all analog expo converter circuits. By doing so, it also eliminated the need for a tempco in the circuit, and produced a more stable circuit.
The uA726 is now long out of production and replacement units are hard to find. A number of counterfeits have appeared on eBay and other places, but most counterfeits are easy to spot: the 726 was manufactured only in a round metal-can package, so if the part being offered is in a DIP or other plastic package, it is a counterfeit.
Depending on how the circuit uses the uA726, it may get hot enough during operation to cause minor burns. Be careful working around a 726 in a live circuit.