Fannie Mae’s conventional serious single-family mortgage delinquency rate fell five basis points to 4.14% in May, down from 4.19% a month earlier and from 5.15% a year ago, according to the government-sponsored enterprise’s monthly summary. The multi-family serious mortgage delinquency rate also declined from 0.57% in April to 0.52% last month. As public officials continue to ponder the future of Fannie Mae and Freddie Mac, Fannie’s gross mortgage portfolio declined at a compound annualized rate of 13.5% in May. Last month, the GSE’s gross mortgage portfolio balance stood at $737.8 billion, compared to $746.8 billion a year earlier. Fannie Mae is required to reduce its size further by the end of the year. According to MBS analysts at Deutsche Bank the Fannie Mae maximum allowable limit stands at $729 billion by end-2011.  Of the current portfolio, $419 billion (56%) is in whole loans, $231 billion (31%) is Fannie Mae MBS, $80 billion (11%) is non-agency MBS and $16 billion (2%) is non-Fannie Mae agency MBS. During the month of May, Fannie Mae completed 16,419 loan modifications, bringing the total number of loan mods completed by the GSE in 2011 to 84,133. Year-to-date, Fannie Mae’s end-balance for all mortgage-backed securities in its portfolio stands at $234.674 billion, compared to $287.470 billion a year ago. At the end of May, Fannie held more than $16 billion in non-Fannie agency mortgage-backed securities and $79.3 billion in non-agency, non-Fannie mortgage securities. Also year-to-date, the firm’s total balance on Fannie guaranteed securities and mortgage loans is $2.7 billion, compared to $2.74 billion in May of last year. Write to: Kerri Panchuk.