We have determined the structural organization of the human gene that encodes nuclear lamins A and C, intermediate filament proteins of the nuclear lamina. Sequencing and restriction mapping show that the coding region spans approximately 24 kilobases. The 5'-proximal promoter region contains several GC-rich stretches, a CCAAT box, and a TATA-like element of sequence TATTA. The lamin A/C gene contains 12 exons. Alternative splicing within exon 10 gives rise to two different mRNAs that code for pre-lamin A and lamin C. Consequently, two proteins are generated, only one of which, pre-lamin A, can be modified by isoprenylation. The intron positions in the human lamin A/C gene are generally conserved in the previously characterized genes for Xenopus lamin LIII and mouse lamin B2, but different from those in a Drosophila lamin gene. In the regions coding for the central rod domains, the intron positions are also conserved when compared with the intron positions in the genes for most cytoplasmic intermediate filament proteins except those for nestin and neurofilaments. Analysis of the intron positions in these genes supports the hypothesis that the nuclear lamins and other intermediate filament proteins arose from a common ancestor.