Resistance to thyroid hormone (RTH) is usually dominantly inherited and characterized by elevated thyroid hormone levels together with reduced central and peripheral tissue responsiveness to hormone action. Following linkage of this disorder to the thyroid hormone receptor /3 (TRP) gene locus (l), a number of studies have identified TR/3 mutations in RTH. Cloning of the cDNA encoding human TRPl (2) initially suggested an open reading frame of 456 amino acids. However, subsequent sequencing of this cDNA as well as genomic clones show a guanine rather than adenine at nucleotide position 288, generating a new initiation codon (3) and leading to a predicted protein sequence which contains 461 amino acids. In addition, the exons of the TR/3 gene have been numbered either from 00 to 8 or from 1 to 10, depending on the designation of noncoding exons. Publications to date have used both notations to describe TRP mutations leading to confusion. To avoid this, we suggest that the following nomenclature be adopted henceforth. © 1994 by The Endocrine Society.