Serious delinquencies among US Alt-A residential mortgage-backed securities (RMBS) declined in April for the first time in four years, according to the latest data from Fitch Ratings. Subprime RMBS delinquencies fell in the second straight month, and prime RMBS delinquencies rose slightly. Fitch noted that higher cure rates and an increase volume of loan modifications, along with improvements in both liquidation and roll rates, all contributed to the turnaround in Alt-A and subprime delinquencies in April. "Last month’s improvements may be a signal that RMBS performance is beginning to the turn the corner," said managing director Vincent Barberio, in an e-mailed statement. "The next few months will be a better indicator of whether we’re witnessing the beginnings of a legitimate turnaround or a short-term seasonal effect of tax-refunds." Alt-A RMBS delinquencies slipped 30 basis points (bps) to 34.1% in April from 34.4% in March, Fitch said. It's the first month-over-month decline since April 2006. Subprime RMBS delinquencies fell again in April, down 110bps to 45.2% from 46.3% in March. Despite the improvements, 8% of current Alt-A loans and 35% of current subprime loans have been modified and therefore have risk of re-defaulting. Additionally, prime jumbo RMBS delinquencies rose to 10.2% in April, from 10.1% in March and 5.4% one year earlier. Write to Diana Golobay.