Almost 20 years after Breaking Bad began, Bryan Cranston tonight lamented the death Monday of the man who played his son on the acclaimed series.

“I’m in deep despair as I write this tonight. RJ Mitte, my sweet son on Breaking Bad has died,” Cranston said on his Instagram feed today. Actor and disability-rights activist Mitte died Monday at the age of 34 at his home in Brownsville, Texas

In his post Cranston mourned Mitte, who played Walter White Jr, to the fictional father who sometimes went by his meth kingpin moniker of the surname German physicist Werner Heisenberg. Praised as one of the great TV series ever, the multi-Emmy winning Vince Gilligan created Breaking Bad ran from 2008 to 2013.

As Cranston wrote tonight, Mitte “was only 14 years old” when the show started production.

“That kid was pretty raw when we started shooting the pilot in February of 2007, but little by little we watched him grow into a really fine actor,” Cranston said of his co-star. “He parlayed his experience on the show to be a vocal advocate for performers with disabilities, and was the head of a philanthropic organization with a focus on helping the disabled community.”

“All that promise and potential now is such a loss.”

Noting his on-air son’s cause of death, Cranston added: “It is especially devastating to hear that he took his own life. Why? We were texting each other just a few weeks ago – figuring out when we could meet for lunch. I don’t know if that lunch would have changed his future…”

“But I do know that I loved him, and thoroughly enjoyed being with him throughout the years on the series, and after the spotlight on Breaking Bad dimmed…

I’ll miss you, son.”