Nuclear lamins are intermediate filament (IF) type proteins that form a fibrillar network underlying the inner nuclear membrane. The existence of multiple subtypes of lamins in vertebrates has been interpreted in terms of functional specialization during cell division and differentiation. The structure of a gene encoding an A-type lamin of Xenopus laevis was analysed. Comparison with that of a B-type lamin of the same species shows remarkable conservation of the exon/intron pattern. In both genes the last exon, only 9-12 amino acids in length, encodes the complete information necessary for membrane targeting of lamins, i.e. a ras-related CaaX motif. The lamin A specific extension of the tail domain is encoded by a single additional exon. The 5' boundary of this exon coincides with the sequence divergence between human lamins A and C, for which an alternative splice mechanism had previously been suggested. Arguments are presented suggesting that B-type lamins represent the ancestral type of lamins and that A-type lamins derived there from by exon shuffling. The acquisition of the new exon might explain the different fates of A- and B-types lamins during cell division.