64-bit builds were introduced in the 4.0 release.[1] The 46.0 release replaced GTK+ 2.18 with 3.4 as a system requirement on Linux and other systems running X.Org.[173] Starting with 53.0, the 32-bit builds require the SSE2 instruction set. Firefox also can run on number of other architectures on Linux, including ARM, AArch64, PowerPC, POWER, Sparc, HPPA, MIPS, s390, and in the past Alpha, IA-64 (Intel Itanium) and m68k.