Nucleocapsid protein (NCp7) of human immunodeficiency virus type 1 is found covering the genomic RNA in the interior of the viral particle. It is a highly basic protein with two zinc fingers of the form CX2CX4HX4C which exhibit strong affinity for a zinc cation. To study the structure-function relationship of the N-terminal zinc finger of NCp7, this domain was either deleted or changed to CX2CX4CX4C. We examined virus formation and structure as well as proviral DNA synthesis. Our data show that these two NC mutations result in the formation of particles with an abnormal core morphology and impair the end of proviral DNA synthesis, leading to noninfectious viruses.