The Dow Jones Industrial Average experienced its largest ever one-day decline Monday, taking down Valley company stocks in the process. The Dow lost 1,175 points, a 4.6 percent decline Monday. It was the largest point loss in history, although the index has suffered worse percentage losses in the past. Among the largest capitalized public companies in the Valley region, Walt Disney Co. in Burbank declined 3.7 percent, Amgen Inc. in Thousand Oaks lost 6.6 percent, Avery Dennison Inc. in Glendale fell 3.3 percent, American Homes 4 Rent Inc. in Agoura Hills dropped 3.3 percent and Teledyne Technologies Inc. in Thousand Oaks suffered a 4.6 percent loss. The selloff particularly impacted small biotech companies in the Valley region. Sienna Biopharmaceuticals Inc. in Westlake Village, which went public last July, lost 6.8 percent; Endonovo Therapeutics Inc. in Woodland Hills declined 7.1 percent; and HemaCare Corp. in Van Nuys lost 9.6 percent. The Nasdaq, where most of the Valley region’s biotechs trade, lost 273 points or 3.8 percent Monday.