LOS ANGELES — Gladys Horton, the lead singer of Motown girl group the Marvelettes whose hits included "Please Mr. Postman," has died at the age of 66, her son said Thursday. Horton died Wednesday in a nursing home north of Los Angeles where she had been recovering from a stroke, said her son Vaughn Thornton said in a statement released by the Detroit-based Motown Alumni Association. The singer was only 15 when the Marvelettes released "Please Mr. Postman" in 1961