The Hong Kong Certificate of Identity (CI) was a formal travel document and passport, issued by the Hong Kong Government's Immigration Department until 30 June 1997 (See transfer of the sovereignty of Hong Kong). It is no longer possible to possess a valid CI as a travel document, as all CIs have expired by 30 June 2007, though most CI holders should be eligible to hold the HKSAR Passport. Eligibility Certificates of Identity were issued to Hong Kong permanent residents who did not hold and could not obtain any valid travel documents for overseas travel. For example, permanent residents who were citizens of People's Republic of China (PRC) could obtain neither their Chinese passports unless they held hukou registration in Mainland China, nor their British passports unless they naturalized as British citizens or British Dependent Territories citizens tied in Hong Kong. Such persons could be issued CI in lieu of the national passport. CI were not used to enter the PRC (Mainland China) or Portuguese Macau, and other means of travel documents and/or entry permits were required. Normally, CI were valid for 10 years. Since establishment of the Hong Kong Special Administrative Region on 1 July 1997, no more CI were issued. Those already issued remained valid for travel until expiry (which may pass beyond 30 June 1997; as CIs were valid for a maximum of 10 years), none of which past 30 June 2007. CI holders who are PRC citizens are now eligible for Hong Kong passport. CI and PRC citizenship Most (but not all) former CI holders were solely People's Republic of China (PRC) citizens. Many of them were immigrants from mainland China who had acquired the right of abode in Hong Kong. They did not co-hold any type of British nationality (See British nationality law and Hong Kong for details pertaining to various kinds of British nationalities). Before the handover, they were not eligible for any kinds of PRC passports. The PRC only issued Home Return Permit for them to enter mainland China only. If they did not choose to acquire British nationality through naturalisation, they could apply for the CI as a travel document from the colonial Hong Kong Government. Travel abroad was very difficult for CI holders, because CI was not a proof of nationality, in contrast with passport, thus rendering CI holders practically "stateless", even when most of them were Chinese nationals. Most countries