Mapping of mouse 5′ HS 4 and 5. Wild-type 129 mice were treated with phenylhydrazine as described, and nuclei were isolated from spleen cells on day 4. A DNase I series was generated, digested with Sph I (S) to completion, Southern blotted, and hybridized with a 5′ probe. The “0” lane shows the expected parent band in untreated nuclei followed by samples having undergone increasing DNase I digestion. Degradation bands map to −22.5 kb, −23.3 kb, −24.8 kb, and −26.1 kb relative to the Ey cap (in parentheses), which we refer to as 5′ HS 4, 4.1, 4.2 and 5, respectively (see text). An additional band mapping to −28.4 kb maps to this region but is outside the range of resolution of this gel. Molecular size standards are marked on the left. The size of the parent band, placement of the probe used (KP middle), and the size of the degradation bands are diagrammed below. Sph I cuts at −14,121 and −30,281 relative to the Ey cap. Mapping of the sites was confirmed by hybridizing with a probe from the 3′ end of the same restriction fragment as well as mapping with additional restriction enzymes (data not shown).