As mentioned above, a permanent resident in the United States is generally treated as a citizen for tax purposes unless his or her residency lapses or otherwise ends. Former "long-term" permanent residents remain liable for U.S. taxes unless and until they formally exit the U.S. tax system via IRS Form 8854. An immigrant not legally admitted for permanent residence (such as a guest worker) generally becomes liable for U.S. taxes on worldwide income after spending a certain number of days in the U.S. within a certain time period, as described in IRS Publication 519.