Twelve transgenic founder animals retaining intact copies of the infectious molecular clone of human immunodeficiency virus (HIV)-1 were obtained. All the founders appeared healthy during a 9- to 12-month observation period. However, transgenic offspring of one of the founders (female #13), died within the 1st month of life while manifesting several symptoms characteristic of human AIDS. To discover why only one transgenic lineage was affected and why the founder animal in the affected lineage remained healthy while all of her transgenic offspring were diseased, we compared the organization of the transgene in the transgenic lineages. Restriction enzyme analysis showed that the founder no. 13 was a mosaic carrying in each transgenic cell four tandemly arranged copies of the infectious molecular clone. All the units of the tandem repeat appeared to be correctly preserved with the exception of the 3'-most copy, which terminated near the start of human sequences that flank the 3' long terminal repeat (LTR). The unaffected founders and their transgenic litter usually carried a high number of copies of the provirus. The 5' terminus of the transgene in the unaffected animals appeared to be deleted or rearranged. None of the 12 transgenic founders carried a single copy of integrated provirus. We conclude that infectious molecular clone of HIV-1 can be expressed in transgenic mice, and that the mode of proviral integration similar to that seen during the retroviral infectious cycle (i.e., a single-copy provirus) may be incompatible with the postnatal survival.(ABSTRACT TRUNCATED AT 250 WORDS)