Maura Tierney

Irish-American Maura Tierney has left the cast of NBC’s “Parenthood” because of scheduling conflicts with her ongoing treatments for breast cancer.

The “ER” actress was cast as the lead in the series, which is based on the 1989 Ron Howard movie of the same name, and NBC had already pushed back the drama from fall to midseason to give the star time to undergo treatments.

But the network announced on Thursday that though Tierney is expected to recover, her treatment plan conflicts with the production schedule, and she had no choice but to leave the show.

"While we are saddened that Maura Tierney won't be able to continue in her current role in 'Parenthood,' our main concern is for her quick and full recovery,” NBC said in a statement. “Our thoughts and best wishes are with her.”

Tierney’s spokesperson told TV Guide: “She is deeply disappointed that she will not be able to rejoin such a highly respected, talented, fun and funny group of actors and wishes to thank Jason Katims and NBC for their patience and support.”

“Parenthood,” a dramedy about a large dysfunctional family living in Northern California, had received early praise from critics, largely due to Tierney’s strong performance as a single mom struggling to raise two teenagers.

NBC will recast Tierney’s role in the series, which also stars Peter Krause, Craig T. Nelson, Dax Shepard, Bonnie Bedelia, Monica Potter, Erika Christensen and Sarah Ramos.

Reports say producers are hoping to cast Marisa Tomei in the 44-year-old Irish American’s place.

“Parenthood” was already surrounded by drama last spring when NBC executive Nora O'Brien died while on location for the pilot shoot for the show. Filming was shut down for two days.