The sex chromosomes of the plant species Silene latifolia (white campion) are very young (only 5–10 My old), and all 11 X-linked genes so far described have Y-linked homologues. Theory predicts that X chromosomes should accumulate a nonrandom set of genes. However, little is known about the importance of gene movements between the X and the autosomes in plants, or in any very young sex chromosome system. Here, we isolate from cDNA a new gene, Slcyt, on the S. latifolia X, which encodes a cytochrome B protein. We genetically mapped SlCyt and found that it is located ∼1 cM from the pseudoautosomal region. Genes in this region of the X chromosome have low divergence values from their homologous Y-linked genes, indicating that the X only recently stopped recombining with the Y. Genetic mapping in Silene vulgaris suggests that Slcyt originally belonged to a different linkage group from that of the other S. latifolia X-linked genes. Silene latifolia has no Y-linked homologue of Slcyt, and also no autosomal paralogues seem to exist. Slcyt moved from an autosome to the X very recently, as the Cyt gene is also X linked in Silene dioica, the sister species to S. latifolia, but is probably autosomal in Silene diclinis, implying that a translocation to the X probably occurred after the split between S. diclinis and S. latifolia/S. dioica. Diversity at Slcyt is extremely low (πsyn = 0.16%), and we find an excess of high frequency–derived variants and a negative Tajima's D, suggesting that the translocation was driven by selection.