The parents of are asking for the public's help in finding their diabetic son who has been missing since Wednesday morning.

David Mark, 24, is described as a white male, 5'5" with dark hair. He was last seen wearing a waist-length leather coat, a tan knit shirt (no collar) and black and grey Adidas sneakers. Choking back tears in a press conference earlier this afternoon, Robert and Eileen Mark said their son, a Type 1 diabetic, has not had his long-acting insulin since Tuesday night. Although he may have short-acting insulin with him, he cannot usually go more than 24 hours without the long-lasting insulin and it is likely he is in a confused state without his proper medication.

"We just want to make sure we're not running out of time," Robert Mark said. The Albany family is also saying that David could appear disheveled after a few days away from home and without his medication. He is also diagnosed with Asperger's syndrome and his facial expressions and gate may seem a bit odd at times, his mother said.

Police say witnesses have seen David's car parked on Middlesex Road in Chestnut Hill near the Brimmer & May School for three days. The car is less than a mile from where David's sister lives in Chestnut Hill, his parents say, but he has not contacted her at all.

After David left his home Wednesday morning, his highway transponder (used for tolls) was last pinged at Exit 14 on the Massachusetts Turnpike.

Police said he was then spotted by a waitress at Boston Beer Works near Fenway between 1 p.m. and 3 p.m. Wednesday, who said David "appeared to be in good spirits."

The missing man's father said he and his wife found an open website to the Boston Beer Works on David's computer, leading them to fax a photo of their son to the bar asking if any waitress had served him. David's cellphone was then pinged around 6:40 p.m. at the Charlestown Navy Yard, police said. However, police said this afternoon that the phone could have been within a four-mile radius northeast of the navy yard.