Ultra-High Speed (UHS) designation is available on some SDHC and SDXC cards. UHS-I cards, specified in SD Version 3.01, support a clock frequency of 100 MHz (a quadrupling of the original "Default Speed"), which in four-bit transfer mode could transfer 50 MB/s. UHS-I cards declared as UHS104 also support a clock frequency of 208 MHz, which could transfer 104 MB/s. UHS-I is the only class for which products are currently available.