The expression of Tetrahymena surface proteins serotype H3 (SerH3) and serotype 
T (SerT) is under environmental regulation. SerH3 is expressed when cells are 
incubated between the temperatures of 20 and 35 degrees C, while SerT is 
expressed when cells are grown at temperatures above 35 degrees C. Using a SerH3 
cDNA clone as a hybridization probe, we determined that (i) the SerH3 gene is a 
member of a multigene family; (ii) most members of this multigene family are 
variably rearranged during macronuclear development; and (iii) the gene which 
produces the SerH3 mRNA is reproducibly rearranged during macronuclear 
development.