Isolation and characterization of a sea urchin hsp 70 gene segment. Three clones containing Paracentrotus lividus sea urchin DNA sequences which cross-hybridize to Drosophila heat shock protein (hsp) 70 gene were isolated. The sequence arrangements in the three cloned DNA inserts were compared by restriction and cross-hybridization analysis. The results showed that they contain four different genes related to one Drosophila hsp 70 gene. One of these genes was subcloned, and two of the isolated fragments were shown to hybridize to genomic DNA and to RNA from heat-treated sea urchin embryo.