The HYDIN gene located in human chromosome band 16q22.2 is a large gene encompassing 423 kb of genomic DNA that has been suggested as a candidate for an autosomal recessive form of congenital hydrocephalus. We have found that the human HYDIN locus has been very recently duplicated, with a nearly identical 360-kb paralogous segment inserted on chromosome 1q21.1. The duplication, among the largest interchromosomal segmental duplications described in humans, is not accounted for in the current human genome assembly and appears to be part of a greater than 550-kb contig that must lie within 1 of the 11 sequence gaps currently remaining in 1q21.1. Both copies of the HYDIN gene are expressed in alternatively spliced transcripts. Elucidation of the role of HYDIN in human disease susceptibility will require careful discrimination among the paralogous copies.